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Was the boilers riser size reduced ? I usually always do a 3 inch riser n header weather standard or drop. Usually I do this to guarantee the dryest steam is being produced which out dry steam the system never really performs as it should ,it will work and most due ,due to steam being very forgiving . Was the Honeywell pa404 pressuretroll removed n replaced w a vaporstat , was a 0-3 lb low pressure gauge installed or gauge in ounces installed . With out knowing the pressure you operating at it leads to guessing . All older system usually work better at low pressure and on old two pipe it usually helps ,hi pressure is going to show every bad trap leaking supply valve and nut . Low pressure is your friend . I know most guys would give first born over purchasing and installing a vaporstat on there dime if they didn’t include in pricing . Over the many years of being in the phvac business I ve learned one major thing when it come s to steam never be cheap on your near boiler piping and unless it under 100 k input always do over the minimum suggested header size ,on peerless over a 4 section it’s both tappings and no matter what 3 inch riser usually dry steam and mim issues . All aside was this boiler cleaned the old fashion way ,flushing ,wanding ,steam skimming and re flushing . Dirty water is in my eyes the most over looked issue being everybody excepts chemical to do it all and it doesn’t ,aside usually from what u see bad piping under sized headers and improper if no drips where there should be . Dry steam is what a steam system is looking for not wet and not at a high velocity . These are things piping do . Just some suggestions , low pressure also helps keep the steam out of the returns but is not going to fix bad traps but will stop water from being blown out of water seal type traps all food for thought .
Peace and good luck clammy

You are quite correct. There should be no pressure -- and certainly no steam -- in the dry returns. This suggests, though, that there are two separate problems. One is that somehow steam is getting into that dry return. A connection which shouldn't be there, a failed open radiator trap, a pair of drips to a wet return which isn't actually wet (too high -- above the water lien), a blown water seal (too much boiler pressure)... several possibilities.

It is likely that the presence of steam closed the main vent on that dry return which allowed the pressure buildup-- but... is there one? Is it functioning?

Go back to basics and take a look at everything -- and think like steam (which should be in the mains and radiators and nowhere else) and air (which has to get out of the radiators and then out of the dry return) and water (which has to get out of the radiators, into the dry return, and then through a drip to the wet return or boiler.

What size boiler ? Is that a 3 inch riser or was it reduced? Was this boiler skimmed ,from the drop on the equilizer blocking the skim port i would think not . I see no provision where left to wand the boiler or drain all contents out of the boiler . Possible you have some carry over condensate from the boiler being possible dirty . Are you using a vapor stat as a pressure control or still the pa404 . As other may have asked is the a good low pressure gauge aside from the zero 30 gauge . I usually add nothing and just flush wand and skim boilers I ve installed , some times it takes 2 or 3 times to get all the crap out and at last I’ll add surge master when I’m sure it as clean as it can be just for the o2 and for slight corrosion protection and a great indicator of make up water when it ain’t green they have been adding make up . Follow what other have stated and if there using water seal type traps and graduated supply valve then it gotta be a vapor stat and and low pressure other wise you will be just chasing your tail like a puppy . Usually no part of a steam system I replace has any copper except the water line feeding it . Full port tees w bushing and ball valves for flushing and draining other wise it just glam .
Peace and good luck clammy