Thigh straps: I think these were invented by someone who said "damn, that crotch strap is going to hurt!" but the bottom line is that one simple crotch strap, with one high quality clip (not one designed to hold a belt on your pants) works quite well enough. You just shove the strap off center to your, ah, windward side, give it an inch of slack, and forget about it.
Shock loads? No really, I sincerely hope there's never going to be a shock load on my crotch strap, I expect it only is there to resist the bouyancy pull of my PFD
. Unless someone sends one of those fancy we-can-snatch-em-up-on-a-C130-skyhook rescues for me, I really don't expect any shock load on that strap. (Memo, enquire for vacancies at Venetian Boys' Choir.)