1. Foam pretty much stymies future welding
2. Not sure what the big picture goal is achieved with foam
3. The unibody pseudo crossmember that you are considering filling, is effectively a big arch, in itself is a beautiful structural solution, but that "arch" requires large immovable abutments at each end, or a lateral member securely tying the two ends together to prevent spreading. The second solution does not exist in your pics. A rollbar with a lower lateral tube somewhat achieves that, OR so does a well designed rear DS safety loop assembly, or just a straight lateral connector attached below the DS, which can also tie in the rear seat/seat belt tie downs more robustly.
4. Best to make sure foam has some flame retardant qualities.
5. Those pics are of two different cars?

Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.