My 08 E90 is also a former lease vehicle and so far everything is fine.
You can't over-rev a DCT as the electronics will not let you shift down as long as the resulting revs in the lower gear will be over the rev limit.
It's easy to over-rev a manual car either by downshifting into 2nd instead of 4th for example. Messing up the upshift into 5th and accidently shifting into 3rd is also possible. But anyway, the foot is still on the clutch, so it should only happen to a certain group of drivers who lack understanding and feeling for what is going on with the vehicle.
There is a rev limiter for the downshift as well but since the engine will be driven by the rear wheels it won't help. What the limiter does is to cut fuel-air-mixture supply when reaching the rev limit. That only prevents over-reving under acceleration but not for the downshifts/misshifts.