Mac fan speeds really high
There’s a lot to answer but here are my thinking:


Power off your Mac and reset the SMC one time - those blower speeds are non-sensical and look like an integer overflow.
Run software updates - apply EVERYTHING and consider downloading the OS installer and running it from Recovery HD - install right over your system as that won’t erase any accounts or settings and it will download EFI / firmware updates as needed - especially if you’re connected to a viable network when you reinstall.
A temperature of 100 C isn’t ridiculous - modern CPU run at the boiling point perfectly fine for extended periods - 43 C is a cool fall evening with a fresh breeze and nothing to worry about at all.
Don’t put much stock (if any) in the diagnostics - that’s designed to detect major component failures - not any sort of battery or fine grained test in my experience. Even the tests Apple runs at the Genius Bar while you wait are pretty trivial, basic tests.
Only 3 cycles on a battery makes me think you have a new Mac - be sure you contact iStat menus to be 100 % sure the software fully supports your specific model. I have seen new machines report nonsense until the vendor updates their software for new hardware and new sensors.