You could refill the system with R134A with a dye in it, like duPont "SUVA". If you look under the hood at night with a blacklight this is very bright and you will be able to detect the leak if present. Unfortunately the leak is most likely to be in the evaporator. This is hard to visualize with a blacklight unless you remove the heater fan (and maybe you still won't see it). I found no fluorescent traces anywhere under the hood and concluded by elimination it was the evaporator.

I think the bulk of good advice would tell you not to put any leakstop substances or any other foreign materials in the system. If the system has to be opened up and serviced later the contamination would be very hard to deal with.

The Volvo system like many modern systems is a "cycling clutch orifice tube" system and is quite standard - would not require a Volvo specialist to work on.

I've learned that many cars have a/c evaporators and/or heater cores that are very difficult to get to.