Some Designer vertical rads can take days/weeks for all the air pockets to come out and the rad to heat evenly all over. Try turning all rads off with just that one on for a few hours and see if it shift it
if you are just changing the one on the right drop the pressure to zero and you will be able to change it without having to drain whole system you will get a bit of a dribble as you do it
you are allowed to pump up to 12 litres a minute so jet washers should be set for no more than this
power showers are not connected to the mains they are fed from tanks
mate suffered this every time neighbour used tap fitted simple shock arrestor straight after stop cock and it solved it
https://www.screwfix.com/p/flomasta-vessel-shock-arrestor-1-1-2-bsp/905fa
you wont cause a leak just break the bits off as none of the box forms any sort of seal with the gas service .
The black ring is possibly just like a big circlip holding pipe to box
Then you need a mark2 surface box when replacing it...
cylinder stat is also placed far too high on the cylinder and temp is low at 50
Filter on expansion pipe ?
Not the clearest of photos but it looks like your pump is on the flow into cylinder
No solution to your problem but in the hundreds of plates i have had out changed/cleaned etc i have never seen anything that resembles how gritty that looks . In one side of the plate let along both sides . Very strange
there is an auto vent on top of the pump usually screwed down
Noise on the instants on pre heat was usually a whistling sound associated with the divertor valve