Well, if you have tried all the above, then thoroughly clean AND check all the contacts/slots on the mk3 as well (for bent pins, tiny obstructions, etc.). You can clean the 'gold fingers' with a q-tip and a tiny amount of isopropyl alcohol.
If none of this works, then yeah, it could be an issue with faulty hardware. The mk3 flash zone probably has the best compatibility of all (you shouldn't be seeing a lot of white screens), and something should at least boot from the SD card. It's still a little strange that other functions work perfectly.
Just for the heck of it, you can try testing with a different SD card that's compatible (borrow a Sandisk from a friend or something).
If you decide to return/exchange the mk3, you would need to contact the store you purchased it from...
That's all I can really think of at the moment...
BruceG