By the way, in WoG (and consequently VT), the elixir restored 100% of health. I wonder why the WoG developers decided to make this specific change?

Also, there are units like ghosts and trolls that restore health. I wonder what regeneration is written for them: 50 hp or 100% (well, if a troll suddenly had 100 hp)

Technically, regeneration for everyone works as 50 hp. That is, for example, with the elixir, a troll will not regenerate fully.
Apparently, in RoE, the developers made these 50 hp as a number clearly always greater than the health of ghosts, so that they would always regenerate completely. And then, in AB and SoD, when they added trolls and the elixir, they forgot about this feature and didn't change anything. Most likely, this can be called a bug.
Accordingly, in WoG, they may have decided to fix this bug. Why this one specifically, I don't know; it's not hard to fix, and apparently someone noticed it.