So my happy ending was too good to be true… She started freezing up again (zero flow) while pulling shots or flushing. I dug back in, took her apart, ordered some special tools, somehow managed to get the boilers out (how in the hell will I get them back in???), and, after weeks of effort, I finally pulled out the valve system. Sadly, the boiler check valve looked pristine. But I did see that the chamber below the boiler check valve was heavily deposited, and there was a lot of corrosion in the system below it.
Any suggestions on my next move to get my poor Vesuvius working again?
(BTW, I now have a full RO system installed so that I hopefully won’t have to deal with this again!)