Iceverge

Strange. Do you have a trunk and branch system or a radial?

Just tape the OSB for airtightness and forget the YBS. Insulate the service cavity with mineral wool. Ideally run the battens at 90deg to the rafters.

There's no magic with dMev Vs PIV. Both are just a method of moving a continuous stream of air through the house. I used to leave the hob extractor on when I went out in an old damp rental cottage we had to dry the place out. It's just a fan moving air after all. In both cases you'll still get condensation if you have trapped pockets of air like behind curtains etc. @TakeshiKovacs as an experiment try leaving a bedroom door open to the hallway, curtains open and the bathroom/kitchen extractor fan on constantly for a night. If you can manage the light and noise. It'll solve the condensation I bet. You could also crack the window open a few mm and leave an air gap to allow some air flow through the bedroom to the extractor. Keep us posted.

Masking tape along the perimeter would avoid the need to scrape it from the floor too.

What is under the timber floor? Has this change only come about since you applied the parge coat?

You could do a hybrid roof too, Or maybe SIPS depending on your span. Both work and reduce the thickness of the roof substantially.

Using the chimney to vent the underfloor isn't a good idea thermally. As Nick says it'll cool the house 24/7. Forget rigid boards, impossible to put in there properly. Instead make some Larsen truss type joist extenders with flanges on bottom to tack to the side of the existing joists. If you could rip a 300mm I-joist in half it would be perfect. The OSB webs will be plenty thermal break. Imaging something like these hanging from below your existing joists. Drop in hardboard to sit on top of the new flanges. Use acoustic sealant/tape/foam to make it windtight. Drop in mineral wool insulation. Glue all joists in the flooring deck as your airtight layer. The process is much like this apart from your Larsen trusses. Cover the ground with polythene if you're worried about moisture.

Insulated metal panel roof. https://www.kingspan.com/ie/en/products/insulated-panels/roof-panels/quadcore-roofliner/

What @Temp said.

I really wouldn't get concerned about inlet and exhaust spacing either. Even if they're exactly beside eachother the mixing will be minimal assuming it's an open wall they're going. Think about how to keep snow off the ASHP. Lots of damp snow getting sucked into it doesn't do much for performance right when you need it most.

I would be much much more concerned with the heat loss due to the external length of water pipes to and from the ASHP.

The numbers will tell you like @SteamyTea has done. ASHPs move vast amounts of external air and drop it by such a small amount that there MVHR and ASHP won't notice eachother. I suspect the microclimate effect people are feeling is very much due to the wind chill from the movement of the air when standing nearby rather than an explicit drop in the air temperature .

Until you take it apart. Knock and rebuild.

Slating from the bottom up is tricky. Other than. Than I don't think it's much of an issue just to slate and batten and felt down over the edpm..

Sketch or photo what's built already and we'll give you a sensible proposal. Vapour driven moisture damage is a tremendous red herring. The volume of transmitted water is absolutely miniscule. The real issue is airborne vapour carried by bad airtightness. This is where you need to focus your energy.