1. Fantasy Portal (or portal fantasy) are things more like Inuyasha or even say the Chronicles of Narina where the MC travels to a different world, but can usually go back to their home one. This is not the MC is stuck in another world and has to build a new life, like in Isekai. I do see the two genres as cousins of sorts.
2. Mercurius possess the concept because it's happened before. It's a cyclical thing.
3. I struggled over this too and the simple answer is logistics. If someone was playing that character and they dropped out, the whole thing would be dead. Besides, I view them more as an NPC and will do my best to make sure they don't take over the story from the other players and that everyone gets a chance to show up. This is highly experimental for me, so we'll see if it crash and burns or not. (If it helps, the main protagonists fighting skills are limited, not because she's completely incapable, but because she has no training or experience, you know, like the average person who isn't involved in some kind of combat training).
4. Most of the chatter is on the OOC page. Sure, a couple of posts in character might be chatter, but they're usually character related and I always put the accepted characters and apps at the top in the first post. It's my first time using a site that has such tabs, so I'm still getting used to the layout.