XRAY - Model racing cars

I found out my front right steel driveshaft is bent. Since the car has only 20 batteries of run time and I have never crashed with it, this comes as a strange surprise for me as I don't know why the same thing wouldn't happen with the new driveshaft? Also, driveshafts do not come in pairs?

No, the shafts don't come as a pair. On a solid axle, especially in the front, contortion is normal after a (longer) time. Regarding the bending, did you check for additional gap between outdrives and shafts? Only just a free movement throughout the suspension traveling is too less as the suspension arms always work a little under running. For sure the all new graphite suspension arms should work much less during running.

How could I affect that gap (only camber?) ? Also, in your opinion, which would be a more durable option? 50mm (which apparently wears the outdrives faster) or 52 mm (which maybe have a bigger chance to bend, since they are longer)?

You can affect that gap by camber, trackwidth and a little by toe in. But it's mainly more like that you set your car as you want and then use the proper driveshafts. You also could use the short shafts as a setup option on a wide car theoretically. If you run a more narrow car most of the time I would use the 50mm shafts. I always have had 1mm to 1.5mm additional gap and that was fine ever since. ATM I run the 52mm shafts (F: steel, R: alu) with 1.5mm shims in the front, 1.5mm (RF) and 2mm (RR) in the rear (geared diff.) with 1° to 1.75° camber. So far it's all ok. You can check that gap by removing the wheelhex and its pin and then moving the shaft back and forth. The only thing is that the front shafts show approx 30° of contortion meanwhile after 2 years but that's not really a problem.