Best to not rush this step, any moisture underneath the flooring will be regrettable. Give it a millimetre per day to be safe- 7 mm thick slc = 7 days drying time - this is probably excessive but to be safe then consider this. Hope this helps!
Common for towel rails to have the same valve at each side.
Think of them both as a lockshield.
Open them both, turn on the heating, and see which pipe heats up first. Treat that side as your 'trv' and the other side as your 'lockshield.
Bear in mind that often towel rails are a good heat sink...
It all depends on how much ventilation there is in the basement.
Is there carpet on top of the plastic?
Check all your drain gullies around the house and drain pipes and position of DPC etc.
If you want to be sure whats going on you should check the moisture content of the concrete now and in...
It's commonly done. My last house had a large opening that had been made much smaller with bricks so that a gas fire could be fitted in. It all came out easily and we put in a wood burning stove
You need to check exactly where the water is leaking out, and then we can advise more exactly how to fix it. Maybe check and send another pic with an arrow
Plan sounds good. Ensure wall plates are secure and wall won't pass any damp onto them. Sleeper walls sound good, just ensure there is no bounce at all in the joists before you commence floorboards and then tiling.
Yes add noggins in 2 places, between the walls and sleeper walls each side for...
10mm safely, getting to 30mmm will be pushing it and risking cutting off the entire bottom strip.
Sometimes at the sides and ends you can see the depth of the softwood end sections.
How about you take part off the top and part off the bottom if you need to cut it down?