how many characters can i fall in love with

anyway, hm. honestly, i'd say i can't really see oda as "being afraid" of dazai at all. oda's remark of dazai's darker nature is just that, the truth. dazai really was a cut above the rest in the mafia in more ways than one. he never failed, his results were exceptional, his methods scarily inescapable. oda says someone as lowly a member as him "can't even imagine the amount of money that was (referring to dazai being responsible for about 1/2 of the port mafia's profits for 2 years) or how many lives were lost as a result". this is a truth and an observation (and also a paragraph written for audience benefit). let me just try and give my interpretation/explanations for the scenes you've provided to support my beliefs.

Scene A: Dazai in the Freedom Café

i think your interpretation might be reading a little too much into the joke? oda says "in a few years, they might be able to hold up a bank", to which dazai says "maybe i should recruit them, then". i don't really think there was any specific malice from dazai's side here, if that's what you're implying? like, he isn't trying to scare oda or something by coming into the kids' residence and talking about recruiting them? it was just a silly follow-up to oda bragging about the kids' criminal capabilities.

naturally, oda wouldn't actually want that for the kids. he's vehemently opposed to the kids landing in a situation where they'd be going up against Ability users even in BEAST, where kousuke said he wanted to join the ADA like oda. that being said, if oda really felt uncomfortable about dazai knowing about the kids in the sense of "he's a danger to them", please understand his priorities: there's no world in which oda would prioritise anyone over the kids. i fully believe that there's no stone oda would leave unturned to ensure the kids' safety, let alone be deterred by the cost of talking in a manner that he would consider "improper" given dazai's status. also, the café is literally under the port mafia's jurisdiction.

one thing to consider here as a component of reasoning as to why odasaku would be "uncomfortable" regarding dazai knowing about this is the fact that he's a very closed-off guy. he says it so himself in TDIPUD, he's a moody person. he seems to be uncomfortable about people knowing details of his life in general: he doesn't like talking or even mentioning his no kill policy, he stays shut about his past, he's really just. like that.

about him repeatedly diverting the conversation, i'd say it's a culmination of factors that make him anxious: dazai coming somewhere unexpected of him to be + the fact that their case, last he heard of it, was implying that they might lose ango. there had to be something pressing for dazai to have turned up here and he wanted to get to the root of it. so, in my opinion, he isn't anxious about dazai having turned up there, he's anxious about what could have caused him to. he doesn't seem to be in any particular hurry to get dazai out of there either, they do, after all, indulge in this nonsense:

however, let's try going with your given interpretation of this scene (and please don't think this is me saying it's wrong or anything, just that my interpretation doesn't agree with it): oda is uncomfortable with dazai being at the café because he considers dazai a threat to the children. he repeatedly stops small talk to get to the point because he needs dazai out of there as soon as possible. if this is true, then it doesn't really fall in line with what happens next when dazai seems to have a fifteen moment™ about ango next:

(and then they get cockblocked by a phone call.)

had oda been so wrapped in his fear of dazai, he wouldn't have tried to pry. this also leads me to answer your last point: would oda have just left dazai in the mafia without a word? given this paragraph and his increasing concern for his friend especially after the alley scene, i don't think he would.

Scene B: Lupin Confrontation

okay, with this i think there's a lot of underestimating how angry oda himself was at ango going on, coupled with the reason dazai himself is angry at ango. the points to understand here are: 1) neither oda nor dazai want ango dead, 2) oda doesn't know what dazai wants with ango and 3) it would, in fact, be dazai's duty to capture ango as a traitor; oda knows only this.

he doesn't speak up in ango's defense because betraying the mafia would mean the kids would be killed, and it would be futile anyway. (this is something that is mentioned in fifteen as well, the family of traitors in the Port Mafia also faced retaliation).

i'm not sure how his supposed fear of dazai specifically ties in here? he makes a weighted decision, and then asks dazai if he was going to make their place of relief into a war zone. this isn't oda and ango vs dazai, this is ango vs the port mafia. oda isn't afraid of "going up against dazai", he's the member of a mafia that he can't betray or will be faced with consequences for.

i don't really get the burden of "dazai's just acting like an asshole" aimed at dazai here because wouldn't he just be doing his job though? like, even as a friend, oda can't straight-up tell dazai "hey, i know this is a three-way government spy and you're an executive in the mafia, but can you not take him in?" because dazai would just be fulfilling his duty (a point which is very often levied for ango's betrayal, even though dazai isn't the one that actually goes through with his supposed "duty") and he would have inhibited the actions of an executive that caught a spy. it's very important to understand that they're members of a mafia-a place of violence, loyalty, and strong retaliation. they're not regular citizens roaming about that dazai decided to threaten out of nowhere.

dazai's had to go against oda's wishes in the same novel before (alley scene where they kill the mimic agents), and dazai recognises that oda wouldn't have wanted that and apologises to him even though, again, there genuinely wasn't much else they could've done. oda does ask dazai if the soldiers are dead in a morose tone even though he himself knew that was the only way. so i don't think his fear of dazai was what was playing into his hesistance in the lupin scene here.

the tone of things in this scene doesn't seem to suggest that dazai is, in any way, "at fault" for oda's inferiority complex against him. he always stands before oda humbly and as a friend and equal rather than a figure to respect and fear.

as for dazai's anger at ango, i'm sure it was in part due to his allegiance to the port mafia but that's hardly all of it. dazai himself proves that he would betray the mafia in a jiffy for his friends. no, i think him being so angry at ango is because ango played the pair of them (at least, from dazai's perspective) for two whole years and now he was going to lose the one singular nice thing he'd manage to get his hands on in his frankly miserable life. his little breakdown in the freedom café suggests that it's the fact that he will now have to hunt down and kill ango that really gets to him. i mean, he's been assigned to it, and you do what you're assigned to in the mafia. losing everything he finds worth fighting for is, after all, the norm. he's sooooo okay about it.

and dazai being simply angry at ango for betraying the port mafia instead of him and oda doesn't explain why oda himself was angry (which he was, he bluntly stops ango from hypothesising about a future where the three of them can be together again). oda doesn't have a fraction of dazai's loyalty to the mafia. i think it's better to conclude here that the factor driving their anger at ango was their betrayal of their friendship more than anything else.

so if oda were to sit down and explain to dazai why he wanted to leave, i don't think dazai would "crash out" about it. and i'm inclined to believe that oda wouldn't just leave without a word. if he did, though, it would be because he doesn't consider himself to be someone dazai ought to worry about or even miss, more than fear of what dazai would do. and if he really did leave like that, then leaving "without saying goodbye to the only man who stayed by his side as a friend" would eat away at him forever.

all of this isn't to imply that their relationship isn't flawed at all, but that the causes of the "flaws" lie more within themselves respectively rather than in the other. oda considers himself someone that isn't qualified to advise others (this is, of course, exacerbated by the fact that dazai is significantly higher up than oda in the mafia and he thinks he should respect that) because of his own inability to see himself as worth anything not because of how dazai treats him or what he fears the repercussions of treating dazai like a friend will be. like in this scene:

or here:

he's the exact same way in BEAST too, calling himself a worn-out man that isn't worth anything.

dazai himself is at an extremely unbalanced state in dark era (more so than you see in any other light novel), swinging wildly from suicidal to homicidal in a desperate bid to cling onto anything that will help him live another day. and they both (in fact, all three of them) follow the mafia's rule of never prying into each others' lives till everything topples and comes crashing down.

what causes oda to break out of his passivity is desperation to save his friend and the fact that his life is over, but with time i think that would've happened anyway. what causes dazai to finally confront that his life was going nowhere that he wanted it to go was his friend's words, but had he said those words in a conversation in which they decided to get personal, i think he would have listened anyway. you're right about TDIPUD, and i do wonder what caused the shift in the dynamic, but that might just be the fact that TDIPUD was conceived long after dark era after all. maybe oda became worn down hearing how much potential he was wasting by not killing, his lowly rank, and the fact that the rules of the organisation demanded he treat dazai differently. dazai seems to have grown fonder of oda since the DHC arc as oda went from "a man he finds really interesting" to "my best friend". there was a lot of imperfection, but nothing that couldn't have been tackled. narratively, oda's death caused the activity that would pushed dazai into the light. this is undeniable. but whether dazai would have just died in the port mafia if oda lived...i personally believe that it wouldn't be like that.