1. While you can make multiplayer, you cannot make in-browser games. Maybe there is a dll somewhere. idk
2. No, this is not a modeling program and MED isn't good enough.
3. There are restrictions to the detail. A model can only have so many polygons before it begans to ruin the framerate. Likewise, there can only be so many models before the framerate begans to drop. The more detail, the harder the engine has to work, the slower the game performs.
4. Yes, you will need a good modeling program at the very least to go along with Gamestudio. I use gameSpace, but to each his own. Shaders can be used by 3rd party or programmed via c-script.
5. There are no limits to controlling various body parts. You either make all body parts separately and animate them seperately and use a script to attach them or you animate the whole model, but that would be many animations.

Start slowly. When you program, take it step by step. For example, if I want to make the character walk, I make him move first. Once i've got that down, I add collision detection. I add key commands. I then add animations. And I keep building up until I have my final product.

Also, do tutorials and experiment. Best way to learn.