Why can't Carrow attack while invisible? Would he just become visible when he attacks?
This is a good question. I have an answer, but it's a bit weird. Being rooted in one-session tabletop fantasy (like the original HeroQuest), Warhammer Quest is mostly simpler than RPG systems. And yet, somehow the designers managed to make sure that every spell and item with an invisibility effect functions differently.
When I had Jekaena learn new spells, they were all just regular Wizard spells, and the Wizard invisibility spell explicitly allows any non-offensive actions. You can move, use bandages, cast defensive spells, etc. But you can't fire a bow (and they even use that as an example in the text).
The ideal might be to have Jekaena wait until after Carrow attacks and then make him invisible. Coril could shoot him first, but only for one round. But Jekaena is not Tomas and is not accustomed to magical combat. So she'd have to be told to do that.