I am not an electrician, but I have made many adapters. You can go to Home Depot, et al, and pick up receptacles and plugs and a few feet of wire and make an adapter, or wire your outlet as a 14-50 and make an adapter for your welder. Make sure you use adequate wire size, #6, for 50 amp (40 amp continuous).
Cutting off the grounding pin on the UMC sounds like the wrong way to do this. Most electrical devices check for voltage and ground, and when making an adapter, you need to make sure that both ground and neg wires are hooked up to the ground of the circuit breaker box. If you cut off the pin, that won't happen. And then your UMC probably won't work.
Since you will be using the outlet to charge more than weld, I would make the 14-50 the outlet of choice, and then when you weld, you can plug that adapter in, leaving it on your welder cord.
There are several 50 amp plug types. You might make several adapters. They are relatively easy to do. Then you can charge where ever you might be.