He's right about not using new bitumen in the house - that's no longer allowed.
But bitumen stains on blocks don't do any harm, as long as you don't heat them
Why would the hallway be too narrow for floating??
Do use a thresholds in between the areas, the climate in the hallway will be different than in the larger area. And use an underlayment that doesn't contain a DPM since all your underfloors will be either existing floorboards or sheet material
Your Bamboo floor is too narrow and too short to install floating (by using an underlayment and glueing the T&G.
Knowing the T&G of bamboo flooring I wouldn't secret nail it either, but fully bond the board with flexible adhesive to your plywood underfloor
It also depends on the quality of your wood-engineered boards, especially the quality bonding between top-layer and backing.
For instance, these products are highly suitable for UFH and could tackle the hot-spot - as long as you treat your UFH normally and don't exceed the maximum temperature...