Mr Punter

You often see a course of thermal blockwork for internal walls and soleplate on that. It stops the soleplate being cold and allows for insulation and screed. Soleplate on slab is old school as it is cold and more prone to rot. Also if shotfired they often use l shaped soleplate anchors so as not to penetrate the DPC under the soleplate.

Beechwood Hall in Cooksbridge is clad using Sweet Chestnut. https://www.facebook.com/photo.php?fbid=871128391694956&set=pb.100063934084990.-2207520000&type=3

Slip ties may be better, with a sleeve on one side where you have a vertical movement joint in a panel. For tying to the frame, the normal timber frame ties will be OK.

If you are looking at a home grown timber, Sweet Chestnut is worth considering.

How about fibre cement planks, like Cedral or Hardie? I prefer the smooth finish. They need little maintenance and the finish is long lasting because the planks don't expand like timber. Also they are fire resistant. If you are thinking of fire treated timber you may need to have them pressure treated with Non Com, not just coated. Very expensive.

The top layout is 36m2, the lower is 64m2. Timber frame with blockwork skin would work OK and not too expensive. You would need building regs.

Maybe the shower could connect to the stack separately. The WC flush may pull the shower trap as configured.

18mm OSB should not sag if correctly installed on 400 centre joists / firrings. If the boards are defective, they need replacing. The grp is there as a waterproofing layer, not to make up for dodgy OSB.

Do you have damp in the floor, the walls or both? Where is the external ground level in relation to the inside floor? Have you checked for any possible leaking drains, gutters and pipes? I have seen the silicone rods and they seem a cleaner way of installing a DPC than the liquid, where it was hard to adjust dosage etc. Did yours fail? If so, did you follow the manufacturers instructions?

SIPS OSB I assume.

18mm OSB or chipboard t & g, edges glued together. Leave a gap of 10mm around the edges for expansion.

Half the diameter.

I would use 50mm galvanised coach screws with a galv washer.

Well done for doing the test. Ask the manufacturer why it does not perform correctly. Let us know what they say. They were top of my search results. I would hope they are not diverting the money intended to create a decent product to advertising.

I have had Weber LAC applied with mesh in the past. 2 costs is about 5mil and you can just sponge finish and paint.