Should be fairly easy, but recommend you be competent in electrics. If existing is opentherm then you’d be better keeping it the same, Hive now do opentherm. Wave is Worcester’s thermostat, is this compatible with your boiler?
I don't know why they would do that, but the opening post was about a lack of hot water on an open vent system. As above, if it was open or even passing, fill as many buckets as you like.
I’m not sure why you think he’d have loads in his van. I carry about 2 bottles, but that’s because I work in social housing and can’t be bothered to take a trip to stores to collect, and we now have out of area properties too.
The seal for that opening is what makes it room sealed and is formed by the newer Vaillant by having the rubber seal form part of the pcb housing. If its damaged etc then it becomes At Risk only unless leaking products of combustion/carbon monoxide, then it becomes Immediately Dangerous.