So, how important is it to join a guild?
Well, it just depends on your personality. I play MMOs to be around different people, but in no way what so ever do you need a solid group of people while leveling.
When it comes to end-game, there's three ways to get gear: PvP (where you kill people), PvE raiding (where you'll need to work with 23 others to fight bosses), and Tradeskilling (where you farm up mats to create items). Just depending what you want out of the game, you can solo for the majority of it. As far as AoC goes, their raiding is the best thing about the game. So unless you're 100% against working with others, I'd join a guild, level up with others, and get into raids ASAP.
Can you solo? Is it mission based and are the missions instanced, or can horrible reprobates hide in the shadows and wait for you to beat down the boss to one hitpoint and then jump in for the killshot and get all the credit?
You can absolutely solo.
There are instances. The majority of missions are in a huge zone where other people can be.
The first person to do 1 dmg to a monster will make that monster locked to that person. So if you hit something, no one can kill-steal it from you.
Also how twitchy is it?
Depending on your term of twitchy.
My definition of twitchy at it's most extreme is World of Warcraft's playstyle. I could interrupt a .5 second cast. Most game's animations get in the way of something that quick.
AoC has animations, but if you are melee you can twist your character out of animations making it 100% more twitchy than it normally would be.
Can you play it with one or two fingers?
Depending on what you want to do. My keybindings look like: Q, E, R, T, F, G, Z, X, C , V, B, 1, 2, 3, 4 / shift all of those / control all of those.
I only have that many because it's necessary for a healer to have that much bound. Other classes, like necros/demos have much less bindings. Melee classes automatically have to have 5 buttons bound for the arrow system in the game. (And of course your movement is WSAD).