Parts not covered under extended warranty?

That seems like a lot of money to replace the seat mechanism, perhaps you could try an independent Mazda mechanic and replace it with used parts. That would save you some money. As for the extended warranty, you should read it and understand what is covered and what is not. The warranty is generally not from the carmaker directly, but from an insurance company and sold through the dealer. If it is correct that the failure was due to faulty welding, as you say it was suggested, then it's a problem for Mazda itself to fix.