Overthinking it 😉
I used tiling battery to create the air gap but it also allows you to stick the insulation to the batten with silicone, make sure your insulation is good tight fit between the rafters, use airtight foam on the wall plate etc.
Just wash the heat exchanger in the bath, I’m sure I read somewhere that it has to be fully dry before it’s fitted though?
I had to pack my vent axia unit out a touch at the bottom as I had water coming out of the front but only slightly, this helped the issue. I was shocked at how much water the unit produced in the winter as I was catching it in a bucket until I got the drain sorted.
Mine was in a cold attic & insulated as good as I could get it.
Welcome to the forum.
Sounds like you have researched it well, it’s easy enough to self install. You will need to insulate the unit, ducts & manifolds in the attic.
I can’t help with the calculations but others certainly will.
Ecology.
No issue if you own the land.
6-8 weeks for application, quite difficult to get the first appointment.
You need to have a plan of costs, proof of income, outgoings etc.
Very helpful & easy to deal with.