Hi Muguet! Welcome to the site and I hope I can at least answer some of these ^^
1) This sort of thing is something we usually tackle when we get to it, since it's mostly relevant only when a user gets a character that can evolve into those forms. For a more concrete answer though, it depends a lot on who you're RPing with and what you want the Gothitelle or Xatu to see. In general though, we would encourage you to check with a mod or admin on a case by case basis if you aren't sure, but in general I could probably see something like the following working for each;
For Gothitelle, you would not be permitted to predict the lifespan of another user's character without their permission, since that could be considered a form of power playing, but if you wanted to predict the lifespan of your own character or an NPC that has no bearing on other RPers, then you would be able to freely. For example, you wouldn't be able to predict the lifespans of the Guildmaster NPCs, but you would be able to for a wild encounter within a thread of yours, or for an NPC directly related to your character, such as a parent figure etc.
For Xatu, it would be very similar to Gothitelle in that you would not be able to make blanket future predicitons about the site's world without running it by a mod or administrator first, but could see smaller parts of personal futures without much guidance, with the scale of NPCs you can freely predict being the same as for Gothitelle. Predicting your own character's future would also be perfectly fine as long as you consult with other RPers first if it also concerns their characters.
TL;DR, you wouldn't have to ignore those abilities entirely, but we would put restrictions on how they could be used.
2) I personally would leave a lot of freedom for how to handle this up to the RPer with an Electrode character. However, a potential interpretation could be that Electrode's 'fun' explosions aren't as powerful as actually using the move Explosion, thus it doesn't faint when exploding for fun but does when using the move and trying to do actual damage to an opponent. Concerning fainting, you would be in a thread with another character so theoretically they could heal you. But otherwise, your character would just be knocked out until they woke up again. How long that takes would be up to you and the others in your thread.
3) I would say no for all of these. Of course, if you do want to RP needing a shellder around to evolve a Slowpoke character then that's perfectly fine and up to you. We just wouldn't make it a concrete factor needed since it's a bit of an unnecessary hindrance for the people who don't want to do that. Definitely a hard no for de-evolving though. Evolution in the pokemon world is permanent.
I hope that answers everything, but let me know if you have any more questions!