No avarage gas regulator can cope with liquids.

By using the extinguisher upright, you're feeding liquid CO2 in the Trev.
If you did set it very low, the reg is closed. And won't let the liquid flow trough.
But a tad higher and the liquid will enter your setup. Depending on total volume in the setup, and the function of the low pressure relief valve, you can get ripping hoses or failing low pressure push locks/couplings, damaged valves , or the worst case, exploding buffers. (luckely you didn't have one)

Concerning pressure gauges and Trevor regulators.
The Trevor has 1 1/8 bsp outlet, that's the low pressure side. You'll need a T-piece (or 2) to have a dump/low pressure PRV and gauge on the basic setup.