You can just use a cpu-burner if max. temp is only target for testing. P95 test relationship between certain hardware components and the way you have set them up - by crunching gazillion of calculations. Test much more than heat. A slight mismatch and it fails = setup is wrong. Fix and repeat, fix and repeat... Temperature does go up but just another parameter to count with. Does not matter if parts heat up from "synthetic" test, game or a hot summer day. Heat is heat. P95 also test cooling but temp can change dramatically when big vcard is running full speed, previous P95 test could be redundant

Should not be a problem but depends on how far there is to critical state. P95 is not just about making sure computer run right now, but to make sure it runs under all conditions, in theory 24/7 365 days per year. To keep healthy distance to critical state is the big trick. Longer distance equals lower output from benchmarking - see the conflict?

Well, P95 has limitations regarding heat. Im pretty sure a gaming session can tax cooling setup more than P95 on its own. You also get usb devices working, sound, may be networking. Ever heard of strange usb events at xxx fsb? All connected in mysterious ways = mysterious bugs and errors. Chipset can have specific "features" one must know/research to make the most of options. Like a big puzzle. No chain is stronger than the weakest link so testing is quite a job if really really "stable" is goal
I do know school of 30min-is-enough, 1 level above "screenshot-stable" but good luck

With quality components may be you dont need much but I was never on that wagon for sure, mainly because luck should not play any role with tinkering. Do it right or not at all. Whole idea of testing, and I would add a bit of research and time spend understanding how things work, is to neutralize luck. Luck cant be trusted and bsods, other errors are not an option. Settings will work on hottest summer day as well as during winter. Dont matter which game/program, what is plugged in or active - all perfect. Anything else is unacceptable. How I looked at it anyway.
Oc lost me when makers stop warning against it, now an advertised feature. Progress but boring. Can also be costly, seems like a waste of money now.