Hey there!
My name's Knock and I play Fala'thorei, at the moment the only fully trained Demon Hunter on the server.
1- Is there anyone I could train under in-game? Or will I have to do it entirely forum posts?
Fala'thorei could train you, and soon enough (I think 3 weeks or so) Navren could too. Though, as Danalthar said, you'll need to be convincing, both OOC and IC. As a player, I wouldn't train someone unless I thought they could roleplay a Demon Hunter well. It's a class that's seen plenty of over-use and abuse on live servers so I for one would have to be comfortable knowing you'd be able to pull it off. As for IC, Fala is trusting of some, but you'll have a hard time convincing him if he's just met you.
For an idea of what's involved in forum posts, you can have a look at my prestige thread
here, or Anski (Navren)'s
here.
2 (and perhaps the most important!)- Most Demon Hunters are warriors, correct? If so, Mythor is a hunter. Would it even be possible for him to prestige as such? Or is it a warrior-only thing?
The important thing to note about Prestiges is that once you take on that new class, you essentially drop your old skills. While Demon Hunters are a Melee class, they have a few anti-magic abilities and are particularly strong against demons. If Mythor switches from a hunter to a Demon Hunter, he'll drop most of his abilities IC, such as his skill with ranged weapons and his control of a pet.
3- How isolated does a Demon Hunter become by following this path?
It depends on the player. Demon Hunters are generally shunned by society. I think Fala's only been in a major city once since completing his training, and that was under heavy disguise. The main goal of a Demon Hunter is to keep killing demons, forsaking all other goals. To compromise with this isolation, I've found other ways to get Fala involved with server RP, such as targeting evil guilds that dabble with demons.
4- How long would it take for the physical changes to start to show up?
It's completely up to your interpretation. There are a multitude of changes that can take place. The most basic being hardened skin and buffed-out muscles, but you can feel free to add whatever you want to your character. Fala grew horns and his fangs and claws became longer and harder, but hasn't had any other drastic changes (e.g. cloven hooves, a tail, etc). It's never explicitly stated in the lore what can and can't happen, so feel free to change your character as you see fit. Bear in mind, though, that drastic changes (such as horns / a tail) could mark you as kill-on-sight to guards, if your character becomes too demon-like.
thanks!