They would normally supply you with a brand new cylinder as when it degrades it breaks off and can clog stuff so defo shouldn’t be used once identified
Yes trim down the brass 1/2 thread to match your build up afterwards eg normally make sure the female iron fitting is flush on the front with the plasterboard etc
Ideally need an aav on the boiler pipe as the gurgling you hear is a vacuum sucking the water out of the boiler trap
Or the boiler condense piping in different
So has copper tube
Too many fittings to convert to each end eg hob end you would need a bent female iron and the supplied cooker elbow so 3 joints that end
You will need two eg one system pack eg for hot water and one zone eg screwfix code 246JK
Then a single zone pack for the remaining zone eg screwfix code 673JK