Ok, so I did test all my GPUs with a used mobo and my PSU and they all work, to my relief.

I RMA'd the 2 faulty mobos and I should get an H81 replacement this week. 

I'm obviously going to test the replacement without riser cables first to assess whether it works or not. 
However, if -like I'm now convinced- the riser actually fried the 2 previous mobos I'm not going to plug them in without somehow testing them beforehands.
I really don't know how, though.
I did examine them but didn't find anything fishy (apart from an apparent saving on solder).

Can I not measure voltages and such with a multimeter before actually endangering my new motherboard?
I can't find anything on the forums on that matter.