I'm sort of surprised that I haven't written a headcanon about this and basically what Nico went through prior to the B.attle of the Lab.yrinth, so I'm going to try to get some things down now. Thoughts are pretty scattered so please accept the bullet points while I feel it out
Nico was most likely 11-12 during most of this, still doesn't have any memories of who he was before the Lo.tus Ho.tel, angry and on his own
(that feels like it's worth saying because it still blows my mind)
I think Nico must have reached out to Hades in some way (unless Hades contacted him), which is how he got a Little bit of filling in on some key lore points; and tbh I wouldn't be surprised if Nico didn't ask Hades directly if he could bring Bianca back, which I'm sure was a conversation that went over well
I think this is also likely when Minos saw/met Nico and decided to take advantage of the situation -- either Hades didn't know or didn't care?? Minos probably overheard Nico asking about bringing Bianca back or just ending up asking him on his own, and then the partnership bloomed from there
So on the one hand, Nico thought that he and Minos were equals, working together towards a common goal, but I also think he recognized in some regard that Minos was using him and Nico thought he could basically manipulate/use Minos in return (he could not)
->Minos definitely fed into Nico's anger against Percy, and also more subtly pushed at Nico's anger with Bianca
->sewed distrust of basically everyone accept Minos. And Nico did recognize it happening to some extent, but he was also so isolated at that point, that he didn't really have anyone else to turn to
I do think Minos taught Nico a lot about the ritual summoning of connecting with ghosts, just the practical bit of what was needed in terms of supplies and then the appropriate chants
Nico also probably learned about Kronos stirring from Minos and who's to say how accurate or not that was
obviously this does a number on who Nico feels like he can trust after being under the tutelage of this ghost for 6 months or so
->partially why it's so impressive to me that he does shift back to Percy and co so quickly-- I think Nico does have generally good instincts about who is trustworthy and that's why he didn't fully trust Minos, even though that wasn't enough for him to not try
->truly Nico's desire to be wanted vs. trusting his innate instincts
anyway, even if Minos is gone, Nico still has nightmares about him
(did hades ever do anything about this???)