No it's the temperature of the room that reaches 12 degrees, the radiator temperature is different so it can reach that target.
Yes, but you could then set the trv to maximum or one below this. Once the trv temperature has been reached then it will stop producing heat output to that radiator...
A trv and room stat in the same room can give poor results, so place the thermostat in the cooler part of the house, eg where it's been mentioned about the hallway and set to the desired temperature, so maybe 15 degrees. Then set the trvs to the temperature you want to feel comfortable at and...
Why is it only temporary? Maybe use a drain bend - finned 4" bend but they're expensive if only temporary. Could use a flexible connection or maker up with a pan connector bend, and an extension connector?
If it’s just been done, why are you left with it? Get them back would be my answer. If you don’t want them back, as there’s already grout and doesn’t appear loose I would silicone seal over that.
Nope. Either utilise the lead, eg a garden tap or needs to be cut back to no more than twice the diameter of the pipe or removed completely (preferred)