At which negative Vce voltage will an NPN transistor be damaged?
This is directly answered in the datasheet right where you'd expect to find it:



The first two lines show you the maximum normal forward voltage the transistor can withstand.  Usually, the backwards voltage is considerably less, which is also the case here.  While the NPN transistor can withstand 50 V collector to emitter, it can only take 5 V accross reverse bias accross the base/emitter junction.  Since applying a reverse voltage from emitter to collector would forward bias the base/collector junction, the base/emitter junction is the limiting factor.

So your answer is 5 volts, plus the B-C junction drop if you want to split hairs (which is not a good idea).

Yes, you could protect this transistor with a reverse diode accross the emitter/collector to clip reverse voltages to safe levels.