Pouring final floor before structure is built

I wouldn't be a fan of many aspects of the build method/order of things outlined but one I certainly would not like my finished floor being exposed to the full construction process it's going to get damaged.

put a secondary layer of dpm in, under the insulation and lap over the block, 1600 gauge polythene or something similar

DPC has forms a tray, which directs water down, but that tray is has a gap to the DPM, so water can pass between the too. Mu preference would be up and over a block or half block. Which is that lapped.

I'd assume that any exposed radon barrier, insulation, or finished floor gets covered with suitable protection during construction. But maybe I'm assume too much of a domestic builder. It certainly should, obviously.

If it's in above the subfloor, you likely still have the ability to go up and over the thermal block. If this meant knocking out a thermal block its worth it.

Surely the top screed is too thin? Whatever your engineer is willing to pass, suggest polishing the floor a few hours after pour, take your time and it’ll come good did it in our build no issues used a scraper and the mortar came off easily.

Putting a separate piece up and over the thermal block is not going to change the flat path way for water to come in.

You need to have a permeant bonded water barrier upstand from the DPM level. A piece flapped on top is not a benefit.

A complete new layer of dpm up and over block would work as suggested.

Anything else is going to be very messy.

You have that flat path from outside to inside.

You could in theory go over the block with a new piece of radon barrier and attempt to seal it to existing at bottom of cavity but there is no way it would be a success and you would never get the 100 percent seal that you would need given the flat water transfer route you have constructed.

Let the radon as installed perform radom duty which it will as it forms a complete barrier under building and across cavity as required and then start again with a dpm where the only purpose with keeping the internal dry.

what does it matter if the moisture runs in between the two layers, its running into an area below the dpm, which would ordinarily be damp anyway, you are overthinking it

no that dpc won't remain in that position, it will be compromised by one of the trades, water will get up under and back to square one, damp proofing only works if the membrane is continuous in my opinion, damp is not hard to prevent but if you try and take a shortcut it'll get you, craft bugger that it is