Well, I solved the jammed handle. That red plastic ring that lets you limit how far you can move the handle to the hot side somehow got moved out of place.
I'm still not sure what causes the hot water flow to be restricted. The passages in the cartridge were clear.
There was a whole lot of crud in the manifold's hot supply passage, which I attempted to remove using q-tips dipped in vinegar. So far it might have helped somewhat. The hot water still didn't flow at first, but the thump and flow happened by itself after a second. It's always worse when the shower hasn't been used in a while, so I'll have to wait and see.