Although it fits on the PSU in the other computer where it burned the PCIe slot, I notice that this molex power cord and the one that came from the bag to that PSU have a different metal prong missing in the PSU end of the plug.
PSU modular connector pinout is not a standard, it helps a lot if you only use PSU's that use same pinout. Even same manufacturer uses different pinouts in different product lines.
do you think the p2 series (750W or higher) can power a 7990 (2x8pin) with two separate pcie cables without frying the cable at some point?
Yes it can, average 8-pin connector needs 300-400W before it gets to danger zone, monitor connector temps when you are doing test runs. Especially if dual mining.