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
not strictly true failure of the gas valve / thermostats and other components can keep a boiler on in fault condition causing the pressure to rise as expansion vessel cannot accommodate the amount of expansion from the heat being generated