Makes no odds to me, IMO the armour only really suffers if they've been poorly installed or subjected to repeated submersion/corrosive gases. Rarely going to get that in domestic unless you fit it too close to the ground.
You could, they normally come in 300 or 500mm so make sure 500 will make it to the tap. You only need to go hand tight + a little nip when connecting to the new tap, don't go mental or you'll ruin the rubber seal
One of these and a 50 to 40mm reducer would work. You'd need a bit of 50mm pipe too. Wouldn't be surprised if there's other workarounds I don't know about though
If we're assuming it's not the boiler and you don't lose pressure the only likely explanation is you're not getting all the air out. Do you have an external pump or expansion vessel? If your filling loop is permanently connected could a passing valve be hiding the pressure drop?
That's a bodge though and not a very good one. It's probably already peeled off the back, insulating tape doesn't really stand the test of time unless it's wrapped tightly around itself
If you can't get compound around the olive you won't get PTFE tape around it. Get a strip of sandpaper and give it the bath towel rub treatment until you can slide the nut back