The radon rough-in shouldn't cost too much. Would depend on the square footage of the footprint. The vapour barrier is required by code either way. I'm actually surprised the radon piping isnt required currently. It's been a code item in the BC Building code since at least 2012.
As for the 3" rigid on the exterior, you should get a detail showing the wall assembly they would be providing.
There are many ways to do that, but I would be concerned about what type of insulation they are using and whether there is a rain screen, as you definitely don't want water getting in between the rigid and the wall sheathing.
PM me if you want to discuss.
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