Mr Punter

Where we are you have to have an accessible ground floor WC with outward opening door that can be made into a shower room. Probably 1.6m x 1.8m. Better not to have the kitchen open to the stairs, so add a door.

We had an Ebac and it really heated the room up which was not good. The Meaco is much better. Less than 200W. I have a couple in a basement rental, with permanent drainage, and they keep the damp at bay.

It is best to seal all the cuts with their touch up paint but I think it is more for aesthetics than durability. With a butt join it would be best to have factory finished ends and join on a support.

We have a Meaco dehumidifier for clothes drying. Very cheap to run and dries brilliantly.

I have done similar but with 600 x 600 tiles 18mm and a pedestal in each corner. Make sure they are non-slip. There is not much room under so you will need to set the pedestal heights and positions spot on as you won't be able to adjust them later. Laser level may be useful. The fillet under the sliding doors looks a bit awkward.

I have used plastic lumber for below ground. Over twice the price of timber and may not have good fire rating for cladding but it does not rot.

Yes, 50mm concrete screws are your friends here. For a 7.5mm screw drill a 7mm hole and clear it out a bit so the screw goes in tight with an impact driver but does not lock up before it is home.

One nail per strap looks a bit stingy. Often see screws and plugs or masonry screws. No issue with zipping off the surplus strap with a grinder and thin blade.

OK in theory but this never ever happens on site, which is why the "plasterboard tents" are notoriously freezing.

In reality this is more likely to lose heat v solid plaster, as it can allow cold outside air to circulate behind the plasterboard. This can be very significant and difficult to resolve. Another failing of SAP.

Unless this is a strict planning condition you may find render is more acceptable to future purchasers and lenders. Probably cheaper and quicker as well.

I don't think you have misread. It looks like a thin coat render onto mesh on carrier board, so probably 6mm thick. God knows what has gone on and what to do. I would go back to the architect and ask for their suggestions.

On the drawing the flashing is not long enough to cover the top row of tiles. Having a roofed canopy addition on a timber frame with a rendered finish is always going to be very tricky to sequence and detail. It looks like the architect put this in the Too Difficult pile and left it for "others" to mess up.

You could use 82mm soil pipe to gain an extra 28mm. Plenty of fittings available. https://plasticsuppliesdirect.co.uk/soil-vent-systems/82mm-push-fit-soil-vent-pipe.html?p=2

You cannot join a WC in a Y. You will need ICs. These are best outside the footprint. Why the soil pipe in bedroom 03? As above, you need to work out your levels and take it from there. Design the runs as shallow as possible with a fall of 1:50.