Depends on the card. Each faction has multiple versions of the leader card. For instance, one version of the Foltest card allows you to instantly clear all weather effects. Other versions have different effects.
She proposed it. It wasn't an official policy though. There's no evidence she was doing it unofficially either. Given that she was very much a "stick to the proper procedure" type, I doubt it.
Actually, some choices do have rather drastically different outcomes that (should) affect the world-at-large. Besides, even small choices can be expanded upon later, or just be referenced, which Witcher 2 did.
Dwarven and gnomish should be completely different from Elvish script, and all three should be completely different from human scripts (except Nilfgaard, which uses a variant of the Elder Tongue). Especially gnomish, because as far as we know, they're the actual native inhabitants of the...
In any case, the person who claimed it was due to the Conjunction is full of it. Sure, we can speculate along those lines, but there are no solid answers in the books about that event. The actual explanation is probably "Because the people who made the comic thought it looked cool and all...
It's worth noting that the Vodyanoi in the game are based directly on the unnamed humanoid fish-creatures Geralt fights in the short story "A Little Sacrifice" in Sword Of Destiny. CDPR just picked a name that Sapkowski later used for something else.