P3D would be your best bet, as a lot of your FSX add-ons may work in P3D. Most aircraft will need to be repurchased, but a lot of sceneries will work as.
This reminds me of the time I sent in a 790FX board that refused to post with any quad core, but would post with a dual core just fine. Got an RMA, and clearly explained what the issue was with the RMA department, and even printed a page with the issue and placed in the box with the mobo. They...
I had a similar issue a few years back trying to run 2 GTX460 cards, and it turned out the secondary PCI-E slot on the motherboard was defective. Try to run one card at a time in each slot to rule out the motherboard.
I wouldn't use the auto overclock feature, as it tends to increase the BCLK above 100, which can cause instability. Leave the BCLK at 100 and tune the multiplier to overclock. 4.5 should be no problem.
IMO small claims court would be a waste of time and money. The other person just won't show up, so you would get a default judgment in your favor. But that doesn't mean the other person will ever pay.
Any time a seller requests a gift payment from me, I tell them only if they ship first. And I always refuse to accept gift payments when I sell items. Not worth the hassle for 3%.