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It's hard to imagine that steam goes down The equalizer. The equalizer is unvented. It would seem like it's pretty much impossible for steam to get past the last main takeoff. A hot equalizer is probably because there is hot water going down the equalizer. And maybe there is steam going up the equalizer

Assume any changes made to the radiator will change output.

However adding a fan behind or below will "force" some convection and add output.

The air space between the radiator and the sheetmetal cover will prevent the cover from reaching the temperature of the cast iron, so radiation transfer is reduced.

Any change to the air flow design through the radiator will change the output.

Now you have a constipated radiator.

Any heat that cannot be "emitted" to the room by any changes, goes back to the boiler.

Agree pretty much with @ethicalpaul . You are about 12% oversized which is not good but is not bad relatively speaking. I'm over 30% oversized but am managing fine. Get the Gorton #2 main vent on, fix the piping to that entryway radiator and either avoid doing setbacks and recoveries, or, if your thermostat programming will allow it break up the recovery into something like 30 minute on off increments. You could also wire in a delay timer which you can find information on in other threads. You want to avoid building the pressure and if you do most of your issues will subside.

Note your main vent isn't going to change alot from a full cold start. It improves time to get steam to the radiators significantly when the system is already warm. Cold, most of the time is eaten up heating the main pipes and condensing steam along the way (steam isn't just going to rush through a cold pipe all the way to the main)

That coupling needs to be replaced with an Eccentric Reducer. The outlet is lower.

Cast Iron, 2 in x 1 in Fitting Pipe Size, Eccentric Reducer Coupling - 4KVZ3|0300154804 - Grainger

There is a picture of it above. It isn't great. The problem really is that there is no place for any carryover to go other than back in to the outlet of the boiler to give they exiting steam another chance at throwing it up in to the main. there is no header but there is a fairly tall riser so it will separate there but the lack of a header means it goes back in to the outlet of the boiler instead of in to the equalizer and in to the return. That actually is probably why it was ok with terrible venting, the pressure was slowing the steam down but now that it is open it can pull water with it.

So the metal casing will absorb but then pass along the heat to the room?