What a beautiful and rare find stateside. Love that shit and lock it up all the time. Get one of those big kryptonite chains or a werewolf
So yes, G3 requires loss of kickstand, and while it is not totally necessary to lose part of the frame, it makes it fit much nicer and keeps it from rubbing.
That variator will work fine with 2 balls in, and will give you a big improvement in acceleration. Do it! If the rattling is just as you come to a stop, that is normal as the balls fall down when the rpms get very low, not enough force to keep them pushed out against the cheek. Lots of noise at higher speeds is questionable.