I too was having trouble riding (road bike) pre-op, due to "step-over" range-of-motion constraints. Your foot is attached to the pedal, and if your rom won't allow you to step over the crank length at the top of the pedal stroke, you end up kind of flailing your knee out - at least I did. I was very fortunate to find a "fitting coach" who knew about ortho issues, and he re-oriented me on the bike (things like you mention: stem height, saddle height, etc.). I also swapped out cranks to get shorter ones (170 cm I believe). I believe the shorter cranks made the biggest difference. Whereas I had to stop riding before the fitting, after, I was able to keep riding right up to surgery. Some 4 yrs post op, one of my standard routes is the "pre surgery" route, which is a shortish, flat 12 mi loop. But I was able to do that the day before surgery.
I recall that it got increasingly difficult to get on the bike, but I could essentially lay it down flat to step over the top tube and then kind of waddle it up.
Keep lookin up, and good riding!
-Dirk