Probably best to let it rest indeed until a future update fixes things....I'm no Linux Guru but I don't mind fiddling around with it, which can sometimes have an adverse effect. On a positive note : at least the public key issue was fixed .
I contacted Brostrend on this issue and unlike Tim I had a fast response and finished up with sharing my screen and letting their tech team sort it out. God knows what they did because my screen disappeared but I got an e-mail a few minutes later which said "The problem was a bad /etc/apt/sources.list.d/raspi.list"
The dongle now works. Although I have no idea what their message means it may help someone on here that understands Linux better than me to sort out their problem.
RPI4 with Stellarmate OS