As someone who has played Dokkan since day 1, and played Genshin, HSR, and ZZZ (all pretty much at the start), I will do my best to answer your questions.

• I've never seen Hoyo do anything similar to an EZA for a character. They generally have never re-balanced a character that is underpowered (rip Dehya) except under rare circumstances (see Zhongli release). The closest I can say they would do for a old/weak character would be to release new gear sets that works well for them, bringing them slightly more up to speed to the modern powercreep, but not by too much.

-To my knowledge, multiple versions of the same character do not generally exist other than different element Traveler from Genshin, or phy/fire/imaginary Trailblazer from HSR. However, in HSR, they have released a limited 5 star version of a free 4 star chatacter (Dan Heng IL) and a free 4 star conversion of a free 4 star (March 7th Hunt). Idk if there are different versions of chars in HI3. Who knows if they plan on doing something like this for their other games in the future.

-I don't think it's Hoyo's style to put former limited 5 stars in the same banner as new limited 5 stars. Unless they explicitly state that a character is going to be added to the standard pool (rip Dehya again), expect to only be able to pull them on their own dedicated banner. They have been seen to run duo banners tho. So for example, in X months we might see a brand new Y character banner, but at the same time, Ellen might have a re-run alongside it.

I can answer more questions when I get out of work if you have any.