Probably not. Treat it as a roof leak and will need looked at from the outside.
Just once which was supplied by the customer. Not really any issues with it.
As above, feed and expansion cistern can get warm but better getting it checked out.
Either way just loosen the the nuts you marked with a red arrow and remove the tap if required. No need to remove the bath panel.
Any particular reason why? Should be fairly simple.
Consider the water regs, embedding pipework is not allowed...
The plaster won't affect it but you should allow for expansion and contraction, especially with plastic.
Afraid so. Those are done with a router and worktop jig.
Have they always been like this?
No, anywhere is fine.
The ones you linked to are traps, nothing else needed.
Looks like an old drain valve for the cylinder.
Used to change these gaskets regularly many moons ago. Not sure if this will fit...
You won't go far wrong with a Mira.
I wouldn't be bothering with iso valves there anyway. The main stopcock is just out of shot (you had a previous thread about the taps).
Use the same thing but a soldered fitting https://www.screwfix.com/p/solder-ring-equal-slip-coupler-15mm/56343
The two chrome valves. Take the caps off, connect the braided hose and turn the black handles till they are in line with the valves.
It should be like that i.e. not permanently connected. Is the braided hose perhaps on top of the boiler?
Separate names with a comma.