OK,
--Windows slow reboot is often caused by the hardware learning mode. My Aurous board allows timing changes after MRC_Done. I'm guessing that MSI boards do the same. I see slow reboot from time to time; especially after changing settings.
--Here's some information about OCCT.
1. OCCT small data set is mostly stressing CPU and very good at producing heat, and exposing problems with cooling the CPU.
2. OCCT Large dataset in "Extreme" mode is good at testing RAM and related subsystems
3. OCCT Power tests CPU, PSU, cooling and memory controller
4. Linpack test can test CPU and CPU subsystems and is reasonably good at testing cooling
Each test is good at a different area, but overlap a bit. Overclocking is an iterative process, filled with critical thinking and differential analysis. If you change too many parameters at once, the process becomes VERY complicated and may become too complex to account for all the different factors at play.
--That is where you are at the moment. You have changed parameters without first fully validating each change. It's hard to think that something that runs a stress test for thirty minutes isn't stable, but in my experience, nothing is stable until I try several different stress tests for several hours. After the first test, I run multiple stress tests at the same time. Even then, gaming on a overclocked system can sometimes find instabilities after all that.
--Windows 10 airplane mode issue that you mention could be ANYTHING, since you've changed so many parameters and crashed so many times. open "Command" using administrator privilege and type:
SFC /scannow
If this comes back with errors you should fix them before anything else.
M.