Time for descent may well have been a certification requirement for max FL - others will know better than I…
I don't believe the ramp schedules were designed to operate far above FL600, I vaguely recall that FL635 was the highest reached during testing but, again, others will know better. A typical LHR-JFK or JFK-LHR would get to between 570 and 590 depending on weight and OAT, LHR-BGI almost invariably ended up level at FL600 due to lower weight and much lower outside temps.
As for reverse inflight, off the top of my head the limits were; Max FL300, speed 250-380kts, max use 4 mins. It didn't make a vast difference to RoD, although it was noticeable.
Inflight rvs was limited to engs 2 and 3 but to enable sufficient air to deploy the reversers, engs 1 and 4 spooled up slightly (in fwd thrust) to help deployment. While this is going on the primary nozzles open fully (for the same reason) but after reverse position is reached, the 2 and 3 primary nozzles have to close to 15% otherwise you get a 'CON' light which means reverse has to be cancelled on that engine (this was not a rare event).
Common sense and airmanship also dictated that you had an escape plan if one or both the engines didn't get the secondary nozzles out of reverse, so you didn't want to do this if fuel was tight.
All-in-all it was far better to make sure you didn't need reverse inflight!