1) So... you're asking what all the renderer calculates then right? Well, depending on what format you choose when you batch render will decide whether or not an alpha channel is included. Targas have these. (and are a high-quality image as well in case you didn't know) Maya's renderer can calculate a z-depth channel, too.

I don't exactly understand your question though. When you render, the only thing you're going to see in the frame is your geometry. Things like lattices, curves, and skeletons will not show up.

2) I myself would recommend creating the cloth and animating it yourself if you've never used cloth dynamics. It can take some time to learn. Maybe someone else can offer you some more insight since I don't animate all that much.