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Ya! you have a condensate drain issue. I would replace the front and rear refractory, do a complete HX cleaning. Remove the refractories before hand. I use CLR cleaner in a squirt bottle and soak it well. Use a stainless steel scouring pad that I get at the Dollar Store to scrub the HX. Use a garden hose to flush the the HX several times until the condesate hose runs freely. I use a feeler gauge to clean between the coils. I also use a stainless wire wheel on a dremel tool and go over the whole HX coils. I bought these wheels at Harbor Freight. Don't use anything but stainless steel to clean the HX. Flush and spray often. It takes me about 3 hours to clean a Munchkin. The longest it took me to clean a Lochinvar was almost 5 hours. Take your time and do it right.

Munchkins need a combustion analysis which is part of my cleaning process.

On low fire the fan rpm is about 1400 and at high fire 3200 as I recall. You have to access the service mode to adjust the rpm up or down. The manual tells you how to access the programing and the code which is 925.

This is what a Munchkin that I cleaned looks like.

Pretty much the air is pushed out of the radiator as it enters — the vent won't close until the steam makes it all the way through the radiator to the vent, so by that time… pretty much full of steam.

You do mention control… and yes, two pipe is a lot easier to control as you can close, or partly close, the inlet to better match steam flow (and thus condensing) to required power. One pipe, as you note or imply, you can't. In most cases one works on a rather intricate interplay of venting rate and cycle time and one can get the balance pretty well. Thermostatically controlled VENTS can also be used — but it's important to remember that the boiler must cycle off and pressure drop to zero from time to time for them to gain control.

cabinet jacks Irwin/Dewalt has inexpensive ones for up to 300lbs. Good for working on doors as well.

https://a.co/d/9L7eG0j