Single pipe system except for one radiator + cycling problem

We have 18 radiators in our three story plus basement building. 17 are single pipe system, only this one is two pipe. It's on the first floor. The feed side comes from above, down into the right side of the radiator, and the return goes out the left side bottom, down into the basement return. This radiator also had an air vent on the left side.

When the control valve was open, it ran fine. Too hot for the room, actually. But when the control valve was off, the air vent spits water that seems to be pushed back up from further down the return line. (We also removed a cap on the elbow at the beginning of the return and it spit water there, too. Like surges of water.)

I know two pipe systems generally don't have vents on radiators, but that some do. I know they usually have vents somewhere else in the steam lines. I suspect someone added the air vent to this radiator before my time, either because they didnt realize the difference, or for better flow. We have since taken off the vent and plugged the radiator, but now the steam flow is a bit weaker/less consistent.

All of this is in conjunction with a short cycling problem nobody can solve. So far we tried a new make up air machine, new thermostat, skimming, flue check