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...
Slowly increase the pressure you will see it climb in 0.2 or 0.3 mbars leave it till at correct pressure then slowly shut off if you go over remove the hose from the fga not the meter stnd run 1 mbar 1 second out / off
Put the tube on while the gas is still on unless it’s difficult or internally