aw. i hope you find your way to mei tai love!
the waist strap shouldn't cut in your waist... is he snug enough so that your shoulders are supporting his weight also, not only the waist?
try doing a bounce while pulling up the straps, just before you tie to get him really snug. that may help with the sagging and waist pressure and twisty shoulder straps.
also, try tying the knot at your hip or your back if you can. or just off to the side a bit, so it's not...