1st question is, yes just plug it in and the GPS mouse will take over.
You do need to update the FLARM every 12 months for it to keep working, so worth checking.
I found the FLARM mouse next to useless. The GPS was not as good as the ones used by PAW. The range that gliders were detected was a lot less than 3km.
If you are in an area that is covered by OGN this is a far better solution? If not, can you get someone interested in ground installation?