Just to make you feel better, at my 6 MONTH office visit, the OS warned me about not complying with the 90* restriction!
I've found a sacroiliac/coccyx cushion from amazon to be just the ticket for adjusting seating in my car and chairs/sofa. Mine is somewhat wedge shaped and keeps my hips higher than knees. Although I don't need it in the house any more, I still use it in the car because folding into low bucket seats isn't optimal for anyone's back or hips.
At my last office visit, my surgeon told me that the most dangerous part about low cushy seating is extricating yourself from it. No quick throwing all the upper body weight forward, and allowing the momentum to help get up. Use a slower more deliberate scooching sort of way to get to the edge of the seat. It's the forceful forward thrust that makes a dislocation risk.
As for restriction time frames, it's obvious that there is wide variance amongst surgeons. My guy is super conservative and it's been wearisome trying to comply. But overall, I'd much rather go slow than go too fast and get into a bad situation. It helped me to comply to realize that the OS has seen a lot more THRs than I have, and he's much better acquainted with the inside construction of my leg and pelvis than I am.
Small comfort, but any port in a storm, right?