1) What class knows the most martial arts? Are they real martial arts like kung fu, or made up ones like krav maga?
If you're a fighter, you get a better base attack bonus, so probably fighters, but that being said there are literally only fighters and magic-users. However if you're a magic-user you could cast strength on yourself and you could play that off as some form of occult martial arts. And there probably aren't any kung fu practitioners on Visterra, unless some humans from our reality got sucked into a wormhole or something, but I'm sure there are various martial arts disciplines practiced both within and outside of the Known Lands.
2) Can I start out having already made a deal with the devil or do I have to do that in game?
Hell yeah you can start out having already made a deal with an archdemon, that's like the coolest backstory for a magic-user! There isn't a "Devil" in Visterra per se, but Lufidiax the deceiver, archdemon of serpents and lies, seems pretty close. Get a little scaly imp familiar dude that cackles constantly and speaks ominously of a future price to pay.
3) Do you want me to write an 8-page backstory? Can I write an 8-page backstory, if I want to? If I write something down in it like I'm the timelost princess of the brass city and the daughter of the sun and I commanded legions in the Hell War but was betrayed by my father's vizier but I don't know that, or that I'm elf conan and cooler than everyone else, will that be true?
You can write an 8 page backstory, but probably no to the planar stuff, elf conan sounds awesome though and you can definitely be elf conan. Please play elf conan.
4) If I eat someone's heart, will I gain their powers? What about their brain?
Maybe if you get afflicted with a weird form of vampirism, but otherwise probably not. If you want to be a magic-user with a spell that grafts their brain on to yours, until you have a big bulbous head full of other peoples' memories, lets talk and see.
5) These classes are boring, can I be one from somewhere else? What about from a different system entirely?
Tell me what kind of character you want to play and if it doesn't fit into the broad categories of "Fighter" and "Magic-User", I'll maybe let you play a new class. Otherwise most classes are just variations of those two with slightly different theming and equipment, and you're welcome to ask me about other possible options and strange equipment.
6) If I make a sword, which one of us gets to name it?
You do, but if you name it something like Stabby McSwordface or something I'll cry on the inside.
7) Am I allowed to kill the other player characters? What would I have to do to be allowed to? Do I win if I kill them all? Actually, how do I win in general?
No you're not allowed to kill the other player characters under normal circumstances, and you'd have to do some pretty incredible co-operative role playing to get me to consider otherwise. You win the shithead of the year award if you kill them all, and you win in general by successfully adventuring long enough to reach Level 5 and then retire.
8) What language stands in for 'Common'? Or what are we all talking to each other in? Like the party, mostly, but also everyone else?
Most folks in the Known Lands speak Low Imperial, which is basically like the Visterran equivalent of middle English. High Imperial is latin, and is only spoken by Syngianite priests.
9) How do I learn how to talk to rocks? No not once a day just, like, normally?
Get cursed by one of the Fae, or find a magic talisman of rock speaking.
10) Which kinds of wizards get to serve kings and live in towers and shit and which ones are run out of town or stoned to death in the streets? Can I be both? At the same time?
If you are friends with someone powerful and have money, you can probably get away with whatever you want and still live in a big tower. However, if you don't have that kind of protection and reputation, you might get run out of town very quickly. People in the Known Lands are kind of scared of magic, as a result of living under a regime which demonized its practiced for such a long time. Despite the Holy Empire of Norenlund falling 200 years ago, people are still frightened of the supernatural. So you can probably be both at the same time.