Perfect marriage I'd say. My family would shoot me if the softener broke down. Depends where you live too. I live in Kent. Our water is 200ppm, That's hard. If you live in Scotland, the water's totally soft. They don't make a boiler for soft water areas, and there are dozens of scale reducing...
What you appear to be holding in you photo is a Bent Male iron to copper fitting probably 22mm x 3/4". The simplest converter is a 22mm x 15mm compression reducer kit made up of three components, The bigger insert goes into your fitting first followed by the olive and lastly the thinner insert...
your suggestion is a good one. You won't know 'til you get the new fire where the flue is or you could google the fire you'd like for the exact specifications but if you can't be bothered it becomes the problem of your fitter. He would sooner work on a completed whole wall starting from scratch...
try turning off all the radiators upstairs.
If its's Y plan or S plan (motorized valves) set programmer to htg only
It can take time, and will probably make allot of rushing when it clears.
If all fails, it's probably the pump..