If you had a poor experience with your current shop with mounting a traditional 2-piece binding, I would be very leery of them mounting a more complicated binding like a Guardian. If the heel base plate is not mounted correctly, it can lead to improper binding function. I would therefore also not recommend mounting a Guardian with a paper jig.
With that said, is this certified employee a new addition to the shop team? Does he bring lots of experience to their staff? Shops quite often learn their lesson and will invest into bringing in more qualified service technicians. If this is what has been done, then I would trust them more.
Lastly, shops do not need to sell the bindings in order to be certified. All they need to do is pass the required tests set by the manufacturers. For example, my old shop did not sell Head/Tyrolia but they were fully certified to mount and test them.