Makes no sense to me. OK, codes and practices differ, but encasing pipework in vertical sheets or piers of concrete seems mad IMO .
What about thermal expansion of the pipework- not inconsiderable for ABS ?
I think you'd be better off consulting a more local forum.
Best wishes.
Its got to be the valve, the tube, or the push button (bellows).
Always worth checking the tube for splits/loose ends.
The valve in the link looks identical to me. If so it will twist off the cradle so would require no further dismantling to swap.
Gas is being burnt @ 8 or 9 kW
where's that heat going? Radiant to the room, convective within the plasterboarded cavity with some convection/radiation eventually to the room as things warm up and lastly chimney losses.
I'd be hard put to it to measure and disentangle these three.
This is a UK site; practices and material ranges differ from yours
That said, try exposing more pipe and using what I believe you colonials term a "no-hub" fitting in the appropriate size as a slip connector...
Oh, but they come up with most creative mumbo-jumbo about "how they work" [i.e. presupposing that do]
but are (ahem) more reticent on the subject of independent trials evidencing that they do anything but sweet FA.
Unvented vessels of up to 15litre capacity are exempted in the regs.
I'd recommend, if you're going for the Ariston 10l on mains water supply, to fit the full safety kit including expansion vessel.
You don't own that meter so not your problem.
The leaks you higlight are all part of the meter installation.
Call your local water undertaking or whoever you pay your water bills to and tell them there's a leak at the (internally located!) meter.