The only way for it to be used illegitimately is if, upon receipt, their technician/s steals the dead HDD and then repairs it at home. The thing is, repairing HDD costs tons of money and makes you literally an asston of money completely legally. We are talking lawer / doctor type money here.
So, you should be about as concerned that your doctor would be stealing your kidney next time you pay 50,000 for a few hours of operation.
I have no idea what type of guarantees the companies in question make though. But if they have the most basic of security and more then 1 (exactly) employee handling RMAs then its fairly safe to say that isn't happening... problem is, I WAS a 1 man RMA department for a company that rolled tens of millions and was selling a variety of products on sears, costco, and a few other major ones (oh, and I handled both USA and canada at once). So it is actually entirely possible for it to be 1 guy who is doing exactly whatever he wants with it.
it is Possible but I wouldn't call it likely. (both the 1 man RMA department and the knowing how to fix HDD and doing it for illegitimate profit)
As for destroying the data without damaging the SSD... you know I am honestly not sure. With a spindle drive I would say degauss (although that would potentially damage newer drives and might be detected as extra damage voiding warranty)... so, maybe someone else would chime in.