It's good that Garmin are sending their own agent to fit the replacement kit, hopefully they will be able to determine whether it was the original kit that was faulty, or if it was a faulty installation by Clipper/Bavaria. If it was working properly on completion of the original installation, it would suggest that the inbuilt splitter peculiar to the Garmin model of AIS unit has failed, thereby allowing full transmission power somehow or other to be fed from the AIS to the VHF Radio and vice versa, knocking each other out in the process. I think it is important that the fitter, or Garmin, if the failed instruments are returned to them, should examine and determine what has happened, as you don't want the same situation to be repeated.
As for fitting a second VHF radio, nothing wrong in doing that, but unless you intend going far in your boat you will probably find that a hand held radio would be quite sufficient for your needs. It's ideal for keeping in your grab bag should you feel the need to sit inside your liferaft, it's handy to keep near to the helm if your fixed radio doesn't extend that far, and it's a a whole lot less money to install.
Good luck with the investigations, and please let us know what the findings are.
