KVM is disabled in kernel by default because almost nobody is using it.
None. Just build a new kernel and enable KVM support and hope it does not break anything else.
So you were basically using the wrong image for the board and wonder that it has hickups?
Support for N2+ is in early development. However you can already build a proper N2+ image using the build tools.
The best and almost only way to properly debug a SBC.
It is a 3pin UART connector featuring RX, TX and GND. If you handle SBCs you defnitily want to invest in an USB-UART adapter. No worries, they are dirt cheap these days.
Enable verbosity and put a serial console to it and check if there is any output to it.
Also create and provide logs with armbianmonitor -u once you rebooted it.
Which image did you use?
Focal and Buster have nothing to do with the architecture. Both flavors are available for armhf (32bit arm) and arm64 (64bit arm).
Btw. running armhf binaries in compat on arm64 is not supported by either one of those.
You never had to use armbian-config. It is just a tool for lazy people and newbies. You are free to do everything by hand. It is included in every Armbian image.
I discussed further differences between the flavors here:
You should decide what to backup first to get an idea about the amount of data it will generated. Then think about the method
Full backup would be dd if=/dev/<source> of=/dev/<target>