How slow could the U-2 fly at 50,000 ft?
You can find the official speed envelope in this copy of the flight manual from '68 that was recently declassified, looks like at FL500 you are right around 105 Kts indicated (convert using figure 2). Also note on this chart it is set up for 1535 gallons of fuel at T.O. the plane had various load configurations (covered in the manual) that would more than likely have an impact on the number,  



(figure 1)

If you want to do the correction to TAS you can use this chart also from the manual 



(figure 2)

Interestingly the copy from '58 that was also declassified (and hosted on the CIA website) is missing most of these charts although it does not look like they were ever in there. 

At higher altitudes the margin was much smaller right around its cruise speed of 373 kts as one of the interesting aspects of the U-2 is that due to its design it was always in and around the Coffin Corner at altitude. According to this former pilot of the U-2 


  When the littler airplane got up to altitude, the margin between the
  climb speed and the critical Mach number and stall became very, very
  minor, and we were looking at plus or minus two-and-a-half knots at
  one particular point in the climb between the climb schedule and
  critical Mach and stall speed so that’s very very small. When we got
  the bigger airplanes, we got up to plus or minus seven-and-a-half
  knots of margin.


This question may also be of interested to you.

NOTE: The above answer is based on currently declassified information about older variants of the U-2. The plane is still in service and has seen more modern variants which include both aerodynamic and power plant changes to the original designs both factors which could effect the above performance data.