1. mavic lockrings have their own threading, so yes. also pay attention to what you're doing and use grease to help you remove the cog and lockring in the future. also make sure everything is tight and check both cog and lockring every so often after riding, epecially after you first install everything. if play develops and you don't retighten everything right away, that's a great step toward stripping... but then again, so is taking off your clothes for money. you be the judge on that one, pal.
2/3. a d-a cog will work fine. pretty much any currently produced cog should work. i have an old campy cog and an eai cog on my ellipses.