I know it's an old thread but I had a single point sling on that mount yesterday. I let it drop a little too quickly and the mount came out of the rear receiver, and the rifle nose dived about 2 inches into the wet mud.
I've had my MP5 apart many times and, I think, the mount needs to be inserted correctly. If the tabs that hold it in had been facing forward, instead of rearwards, I don't think it would have been able to come out, no matter how hard it had been dropped.
I cleared and cleaned the barrel to carry on playing but have just spent a while stripping and thoroughly cleaning the business end of it.
If anyone has a spare sling swivel mount, let me know or if you can point me in the direction of somewhere to get a spare I'd be grateful.