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Thanks all.

The filter is a demineralize filter which goes in on everyone of my boiler replacements.

Also possibly some sediment ended up in the runnout and is creating a dam that is holding water. The internal parts of the valve coming apart also could form a one way valve that lets steam in but does not let water out. If the valve is installed up side down it could potentially not let condensate drain. What does it look like compared to the other valves?

I assume you checked that the valve is all the way open.

Condensate doesn't have to block the pipe to stop the radiator from heating and it usually doesn't block the pipe. You just need a pool of condensate sitting in the pipe. When the steam hits the liquid water it condenses in to water until it has more or less evaporated the water. Steam won't progress beyond the water until it has heated the water to steam temps

Those are Trane in wall convectors or a copy of them.

@EdTheHeaterMan I bought a 24" pipe wrench at HF today. If I can break it free, I'll order the #2 Gorton.

I called up the company and put a pause on everything and contacted a company that works for the city of Beacon which will be honest with us and work in our interest. Thank you all.

What @EdTheHeaterMan sid improper pitch on the branch piping feeding the rad or a defective valve (maybe the disk inside the valve fell off and is blocking the pipe but the pitch is the most likely culprit.

There is probably a low spot in the pipe that fills with water. Over the summer some of the water evaporates and the rad heats at first.

"slopes in the wrong direction"

Yeah, that'll do it.