I haven't watched much of the show, but I did watch the Paris special, and I want to talk about the evil mode, and how it works. As Tikki explains, whenever a miraculous user uses their power, it draws a bit of life-force from the user to fuel it. Normally, this is negligible, however, if a child uses their powers too often, it will cause long-term side effects. The mage who created the miraculous* decided to create protections against overuse (the five minute timer), however he decided to make them get automatically disabled if the user is evil. One wonders why, if he knows an evil-detecting spell that allows him to disable certain functions on the miraculous, he wouldn't set things up so that the user cannot transform or automatically de-transforms. You COULD argue that the spell can only detect if the user is evil after they transform (in the original French, everyone uses the same transformation phrase), and for whatever reason, you can't magically program a miraculous to automatically de-transform the user, but he should AT MINIMUM be able to make it so that the user can't use their powers if they are evil.
Also, even if this was literally the only thing he could do with an evil-detecting spell, it is STILL is a bad idea, because an evil miraculous user being able to overuse their powers is bad for them, but it is even worse for everyone else, and also, the side effects are easily countered by being aware of what is going on, and exercising self restraint. Remember, there is no fundamental difference between Shadybug using her power and Ladybug doing the same, the difference is HOW OFTEN they do it. And Shadybug and Claw Noir used their powers A LOT, at one point Claw Noir uses a cataclysm JUST so that he could leave Marinette's house in a more dramatic way. If you a wondering if Lila is going to run into any of these problems, the answer is "probably not", even if she has no idea this can even happen, simply due to the fact that she doesn't have any reason to use her powers much more often than Ladybug and Chat Noir did before they grew up, she uses it once an akuma fight to create the akuma, and maybe uses a couple of anti-kumas (antikumas?), but she isn't going to be using her powers much more than our heroes did.
Anyway, a mage who lived thousands of years ago being really stupid isn't really important, what is more important is the problems this creates lore wise. Remember, I haven't watched much of the show, so I could be missing something.
How does growing up emotionally speaking increase the amount of life-force in your BODY.
How come you need to feed your kwami after using their power once, if the time limit is there because of the drain on the life-force of the USER.
I would explain this as Tikki giving a simplified explanation due to time constraints, and growing up emotionally increases how EFFICIENTLY you can use life-energy (both your own and your kwami's). A grown adult also has more life energy, but that isn't the main reason they can use a miraculous so many times. Alternatively, a kwami is tired out by a single use of their powers, meaning that after using your power once, you are primarily drawing on your own energy, which adults can handle, but children can't.
* The fact that a mage created the miraculous is from the comics, everything else is from the special.
The best answer I can give you is that Miraculous' lore tends to be comically bad. The timer thing and the existence of evil modes are two of my personal pet peeves. They make no freaking sense!
Re the timer: why would the options be one or infinite? That's nonsense! It makes way more sense for it to be some sort of gradual thing where you get more uses over time. There should also be a cap as infinite uses destroys the tension. When magic powers are plot relevant and a major part of the story, it's almost always better to go a hard magic route verses a soft magic one.
Re the evil mode: what does it even mean to be evil? Who decides that? Tikki and Plagg? And why does being evil only doom children and teenagers? That's weird. It makes way more sense for the power to be neutral things that work the same way no matter who is wielding them especially since evil vs good is largely a matter of perspective.
On to your more specific questions:
How does growing up emotionally speaking increase the amount of life-force in your BODY.
I have no idea. I think everyone was confused when Ladybug and Chat Noir were suddenly adults at 14. It was very weird. Prior to that, the fandom assumed it would happen when they were in their late teens/early-20s. When I do the timers in my own stuff, I just make magic a force in the world and tie everything back to that. For the curious, this is the logic from my lore bible which was written post season three. Even then, I wasn't going to try to make canon's nonsense work:
A miraculousâ special power(s) can be used a maximum of five times per transformation (thatâs five times in combination, not five times per power). The amount of uses will depend on the magical maturity of the Chosen. This number increases slowly from 1 to 5 over the course of magical âpubertyâ. There are extreme fringe cases where someoneâs magic is unusually strong or weak, leading to a limit slightly smaller or greater than 5, but this is very rare.
Humans begin magical âpubertyâ around the ages of 11 to 13 and they reach full maturity around 24 to 26. Humans cannot increase their magic by training, they can only hone it. You get what you get, no more, no less. If someone tries to use a miraculous before theyâve entered magical puberty, no transformation will occur as thereâs no magic to draw on.
How come you need to feed your kwami after using their power once, if the time limit is there because of the drain on the life-force of the USER.
I once again have no idea how this makes any sense. I went with wildly different logic for my version of canon because - back then - I assumed the feeding thing would matter. I still prefer my version to whatever the life force logic is.
Kwamis can only fuel a transformation if their Chosen has magical energy available and if the Kwami has a tie to the mortal world inside of them (aka, food). To make the transformation happen, the Kwami uses the energy from the food to create a bond between Kwami and Chosen. The more magical energy a Chosen has, the stronger the bond, but using special powers weakens the bond. After the bond reaches a certain level of weakness, the bond dissolves and the Chosen is forced to detransform, thus the use limit. Because of this, a wielder can retransform during battle if they feed their kwami between transformations, but there can come a point when even food isnât enough and the Chosen simply needs to rest. What that point is will vary based on a given Chosenâs magical reserves since every transformation and special power use drains a little bit of a Chosenâs personal magic energy. Most of the power comes from the Kwami, though.
Btw, every time the evil mode nonsense come up, I think of a running joke from Barbie: Life in the Dreamhouse where Barbie's closet AI has an evil button for no apparent reason leading to moments like this:
Random evil modes are genuinely funny when played for comedy, but nonsense when played seriously. Why would you ever install that??? And why only punish evil kids but not adults? Seems broken.