Re: SR-71 top altitude
According to this page in the pilot's manual, "The maximum altitude limit is 85,000 feet unless higher altitude is specifically authorized."
That suggests the plane could fly higher than 85,000 feet but normally didn't on operations. It doesn't say why 85,000 feet is the limit nor does it say what the actual ceiling was. The Limit Speed and Altitude Envelope diagram stops at 85,000 feet. The upper curve suggests that at 85,000 feet, the plane's Mach 3.2 design speed intersects the KEAS (knots equivalent airspeed) supersonic minimum airspeed limit.