The PRV needs replacing and the likely culprit is the expansion vessel.
Also it needs cleaning and or a filter fitting.
So now you need a boiler engineer, who will tell you the same, so this middle bit that you are doing now, is pointless.
As above, thousands of flues from basements round here poking out at ankle level, I can't imagine a situation that would cause POC puddling in the basement itself.
safe enough if the PRV is working ok, the vessel might be possible to repressurise, hot water shouldn't be a problem, depending how big your heating system is you may get some function out of it.