2) the engine uses a BSP tree for better and faster rendering and collision detection. but to take advantage of that, the level blocks should all be convex and the level should respect some design restrictions... that means you can't move the vertex of the blocks to wherever you like... actually, making a level that takes full advantage of BSP optimization takes some knowledge of how it works, and it's kind of hard to design your level respecting all of the restrictions it needs to work well.

but you don't need to take advantage of bsp if you don't want to.
you can use models (*.mdl) and terrains (*.hmp) made in the model editor for your level. they have no restrictions to design, so you can make them the way you want. the engine won't be able to use the BSP optimization then, and will need to use a more complex collision detection method for your level, but if you have a good computer, it shouldn't be that big of a problem...


3) many tutorials can be found in links on this forum. try the 'Resources', the 'Wiki' and the 'Magazine' links.

5) making any knid of game takes knowledge and time. as i see it, there are two different ways to start with gamestudio.
if you want to make games for fun, as a hobby, and don't intend to do it professionally in the future, then you could start with the templates, which are pre-programmed pieces of code that you can click together to make games really quick.
if you intend to become a game developer on a more proffessional level, i recommend you stay away from the templates, start small and get the hang of the basics before you go on. try to make a character that can walk around in your level, without using the templates. then make it able to walk and run, then crouch, then jump, then make the camera follow it. and so forth...