The refrigerant with leak sealer is probably safe for use in your car....
however, most shops will refuse to hook their expensive AC recovery machine
to a system containing sealer. If the leak stop doesn't work as advertised,
you may be left with no shop that will work on the system...

I have had some success with slow leaks by pressurizing the system to 100
PSI or so using nitrogen and using soapy water to leak test the joints...
This is, of course after a very close inspection.... the refrigerant will
often carry minute amounts of oil with it and can sometimes act as a
tattletale...

I don't know if new cars work the same as the old so far as refrigerent
and compressors go, but one handy trick for those from the muscle car
era is to use the AC a little bit every month, even in winter
which--according to one mechanic I talked to on it--helps to keep the
refrigerent from leaking out. (I'm guessing it helps keep the seals
tight? Anyway, that was his trick, which he claimed worked well on his
vehicles.)

The correct way for you to check this is to have them draw down a vacuum and
then lock it down and hold it.  Since you have been having this recurring
problem, I would want to see ZERO pressure rise in 30-60 minutes.  If it
can't achieve that there is a leak.  Not a little rise, ZERO rise in
pressure, if there is a little rise, you have a little leak, and next year
you will need another recharge.  often the tell tale signs of the connection
with the leak is the one with the gunk build-up around it.

It really is not rocket science, I have mixed and matched many different
years air-condition components and hoses on the same cars, (92 serpentine
fuel injected system into an 82 body) and sealed them all together with many
years of leak free service.  I have been blessed to have access to a nice
dual recovery /recycle/deep vacuum/charge testing machine.

If there is a leak, the only true fix is to locate the leak and replace the
offending seal.