You do not need a total drain down. Just release the pressure via a rad vent until the boiler pressure gauge reads zero. Then pump air into the expansion vessel. The boiler gauge will increase so you let this out from the rad vent and add more air to the vessel. Carry on with this procedure...
Exactly that. You should only top up the air side of the expansion vessel when there is no pressure on the water side.
If you can feel what you think is the membrane than the air pressure has gone. When was the expansion vessel air side last checked/topped up? My guess would be never since...
In that case you will have to drop the pressure off the whole system by using a system draincock. If there is water from the schrader valve on the vessel then it will need to be replaced. If not then you will probably have to re-pressurise the vessel to the required level (pre-charge pressure...
Air from a tyre is slightly less messy that water which will be expelled if the diaphragm inside the pressure vessel has failed and the system pressure has not been dropped. The off load standing pressure in a heating expansion vessel can be between 1.5 to 3 Bar.
I used my Brother printer to produce labels for the cleats serving various lumps of rope on my friend's 36ft yacht. They stuck like 'STAAB' and are still in in position over 8 years later with no degradation of either printing or adhesion despite being exposed to the elements.
Seconded. Had a customer who thought he would make it easier to get around in his trussed loft by cutting a few 'lumps of wood' out of the way. Cost him half a new roof!!
This site may be of interest. I have used the wedi extruded foam board which was a joy to work with being both very strong, light weight and ready to take tiling without priming. Not the cheapest to be fair but I would have no hesitation in using again when I refurb our bathroom.
Would it not be easier to pull the existing plasterboard off and start again with the backerboard in it's place (assuming you are planning to tile floor to ceiling)? Might help with the tight measurements involved as well.
I bought one of these https://www.screwfix.com/p/roughneck-round-head-long-handled-digging-shovel/243kh to clear out the stream that runs through our garden. It is very heavy and, with a judicial sharpening of the blade on the grindstone, it easily slices through roots including bullrush tubas...