Plumbing – Water supply valve to closed radiant heat system on or off

Your diagnostics so far point to the indirect hot water heater coil leaking potable water into the furnace loop as the most likely issue. You could perhaps verify if you can shut your hot water down for a few days (not convenient, I know.)

Depending on the overall condition of the indirect water heater tank, the coil can often be replaced as a part, if the indirect hot water tank is otherwise in good shape. Some are not designed this way, however.

While your test might leave room for a failed expansion tank, if you did not cool the boiler down too much (so the cool water then expands as it's heated; which with a failed expansion tank would raise the pressure significantly, and with a functioning one will raise the pressure slightly) it's probably valid, especially if the pressure rise took days, rather than shooting up the first time the boiler fired to temperature (a sure sign of expansion tank troubles.)

For a typical "modern" bladder-type expansion tank, there's an easy, quick test of the tank integrity - drop the pressure to zero on the system again and check the pressure on the air fill valve for the expansion tank. If it's also zero, the tank (diaphragm/bladder) has failed. The even quicker (if crude) test is to "burp" the air fill valve very slightly (don't want to release significant air or you need to drop the system pressure and refill it) to see if air or water comes out - if it's water, the tank has failed. I would expect your service technicians to have already done one or both of these tests, given your symptoms.

First of all "because I was told" is a very bad reason to replace something. Next, the house thermostat has nothing to do with this problem. Finally, the hot water tank has nothing more than its own thermostat, so it can't control the boiler temperature.

The high-cut temperature relay may broken or the boiler's thermostat is failing. It's also quite possible that after replacing the expansion tank, you failed to completely bleed the heating system, leading to large air bubbles and quite possibly a failure of the boiler thermostat sensor to be able to read the temperature of the water at the location of the external thermometer you're reading.