I wonder if spraying some silicon lubricant into the belt buckle might free things up. A couple of weeks ago, I was using my leaf blower. I turned it off and then the switch wouldn’t budge so I couldn’t turn it back on. I was up on my first story roof trying to blow out the eaves troughs on the second story–I had tubes attached to the blower. I was desperate to get the job finished and decided it was time for a new blower anyway. I went over to the nearest big box store, but they didn’t have a leaf blower that would accept my extension tubes nor did the store have extension tubes for any of their leaf blowers.
I came home and decided to take the leaf blower apart. I found a lever that operated a slide switch. It was the switch that wouldn’t move. I sprayed contact cleaner into the switch, let it soak for 30 seconds and was able to move the switch. I worked the switch back and forth, reassembled the blower and it works perfectly. In your case, anything with a little lubricant–electrical contact spray, silicon spray, etc. might be just enough to make the metal parts in the buckle slippery so that they can be pulled apart.