There is a new thread on a more current version and installation on raspberry pi located here:
googling MySQL storage location debian brought this up: Where does MySQL store its databases on Debian? - Unix & Linux Stack Exchange
You might want to format the drive with ext4 and mount it via fstab
This seems like a good guide: https://www.theurbanpenguin.com/adding-external-storage-to-your-raspberry-pi/
Question number 3 i have no clue 