JohnMo

Photo, was the best I could find showing a roof light - I have 2 roof lights. They form a cover for the upstand and it all welded together.

Not sure would like that, wife would definitely throw the teddy. We settled on french doors and fixed panel. Ours is 2.7m wide, but could easily be upscaled. We have a lock on the outside, for ease of use. Also just been air tested no leaks.

Switching options are, may be others but not aware of any. Humidistat, but these are not great in the UK climate and humidity swings with seasons. PIR could be with a delay timer, in and out in a minute does nothing, anything longer for example a shower or a number 2 it comes on. Manual switch, our MVHR has humidistat, but is pretty rubbish, so use manual switch by light switch to make boost when showering etc. Light Switching, as you mention no light on, no fan on.

Our roof is at a 12 degree angle, we used EPDM (Sarnafil) which has all seams hot air welded.

Pretty unlikely, you normally don't here anything until or after that date, irrespective of the decision.

We used https://www.realstonecladding.co.uk/natural-stone-cladding-panels Real stone, not composite

Or you could go for something similar to this, with integrated PIR https://www.tlc-direct.co.uk/Products/SLEA4DLC.html

Go to an electrical wholesalers/ trade place, ask them what it is. If they have no idea it a good glue no one uses it, if people do, it will be on the shelf.

If they are building and priced to your building spec, they have a contractual obligation to build to your drawing spec. and to a good standard. The foil missing from plasterboard will allow moisture to pass through the plasterboard, where vapour could condense to water and cause issues. Insulation missing will affect your long term heating costs for the worse. Spilling insulation is poor workmanship.

I used airtight paint. But I would wait until you have screed down. Not sure parge would be very successful, it wasn't when I did mine, hence using airtight paint.

Anyone employed to do work by the builder, has a contract with the builder, you as the end user are not part of the contract in any shape or form. Do as stated above

OK

First the OSB layer has done 90% of your airtightness especially if you use airtight OSB on the joints to worry about. Or if using a membrane a great flat surface to affix to. Your service cavity will have your plumbing and electric stuff within it, why would you want then to make loads of holes for light switches etc in the OSB to allow it to be installed? Seems like a double work effort to me for less gain

Looking at your wall build with respect your service cavity. You have plasterboard, OSB, batten for service cavity. This should be plasterboard, batten for service cavity, OSB. If you floor has a ventilation within the void it's there for good reason. If your insulation is above the ventilation why are you worried?

We had a 1830s sandstone house lathe and plaster walls. Wall about 3 foot thick, still got really hot in summer. No insulation to speak of. But wasn't so bad to heat in the winter. New house has 47 Tonne of concrete in the floor, Durisol walls, be but can still get really hot from solar gain; if you let the sun in for too long. Maybe we get longer exposure to the sun, before it gets too hot,compared to a lightweight house, but difficult to compare.