yes you will sustain the 5.1 audio from decryptor. I believe you can go directly to an mpg from it also, but I'm not sure because i usually skip that step. If not look into vob2mpg.

you can choose to keep the 5.1 audio in gordianknot. you can also just use the original ac3 file to get your 6 wav files (see below)

an ac3 file is the elementary audio file from the DVD. Unless you convert it specifically, it will be in 5.1

besweet is a free audio conversion utility. It can convert ac3 to wav and vice versa. It can also separate your 5.1 ac3 into 6 separate wav files (which you will need for 5.1 editing).

frameserving can be a rather tricky process. I made a tutorial for Vegas here (you have to register to the forum).