I want to use the VPN once as a one-off for my virtual machine but will I be able to remove my VPN from the device and then have 5 total or will it just forever stay there?
with OpenVPN the 5 connection limit is only concurrent connections so you are fine there.
If you put a wireguard key in for the device you can remove the Key again on the website and you are fine as well’
If you use Wireguard, you can manually delete keys https://mullvad.net/en/account/#/ports
Yes, you can remove devices and register new ones.