Yes, you will be able to use 1600 ddr3 ram at the same time with 1866 ddr3 ram. The 1866 ram will run at 1600 with no issues. Good luck with the build and I want to see pics when it's done
What's the system specs your putting it into? Yeah, free shipping back. Well you can just say it didn't work on your PC. Just don't say it wasn't compatible or else they will think it was a buyer error.
1. Check if the motherboard is updated to the most recent bios
2. Make sure it hasn't overclocked itself or something
3. Use your power supply on this PC and see if that fixes the problem
Other options are RMA CPU if putting your power supply in this PC doesn't solve the problem or RMA both CPU and PSU