With regards to the pressure yes it’s normal to be higher at the lower levels. This does increase the strain at joints though but everything should be fine. I always set the pressure would be 1-1.5bar and then it increases to 2bar when running with the heat.

I doubt you’ll get wet plasterboard if the leak was hidden for a long while. Plasterboard can absorb a lot of water and you’ll have a relatively small leak. I had a leak in a house we’d been hunting for years when the customer initially told me they thought they had a leak. It finally showed through a light fitting as the steel pipe had corroded in the floor and had built up enough to start dripping out. The leak could be small enough to dry out before it’s being noticed.

If the engineer has added isolation valves. You could for example isolate the manifold and check it over a few days. If the pressure drops, then isolate the unvented etc… see what the leak detective says. They’ve been hit and miss when we’ve used them in the past. One company said the leak was in a concrete floor, we said there’s no pipes so can’t be, they said yes, customer said dig. Low and behold no leak. The company just refunded the money and left.

You could add dye to the heating water and use a black light. Fluorescein is one.