You can get rid of one of those extensions 100% possibly both by lifting the trap up if the washing machine spigot piece is too long cut it down as you have a removable nut on that one correct ?
Also what’s the kitchen sink plug top / waste connection as that looks like a long one eg could...
Downstairs would recommend as plaster is easier to patch than laminate/ different flooring
Depends are you 100% sure it’s not leaking from above and running down
As for cost 2 hours seem optimistic especially draining down and refilling could take that etc
On the danfoss programmer leave hot water always on eg permanently
As for room stat there maybe a sensor inside size of a tick tack box with danfoss on it with no controls / adjustments just a box with a grill on it this tells the programmer what’s the internal temperature and you set the temp...