The two main bones of contention

[Hitman]

but rather it will be adding the late war technology. Will we be able to add in a schnorkel? ...
What about hedgehogs? Or FOXER devices? Will those be easy enough to mod in?

At the present moment we know alost nothing about how SH5 will be modded, because we yet don't know what will be hardcoded and what not. But I'd like to highlight that if the game stops in May 1943 with the defeat of the uboats, then this is because the allies had by then enough technology to defeat them.

Despite all the things you hear about the wonderful allied technology like Hedgehogs and FIDOS, their use was not as important. It was the extension of centimetric radar to all escorts, and the heavy increase in the number of those escorting ships and patrol airplanes what killed the uboats. If an Uboat had not been killed while exiting the Biscay Bay with a FIDO, it would have been unable to attack a convoy because the radar equipped escorts would have detected him easily, and then would have felt as victim to a hunter killer support group called to finish him by the regular convoy escort. So the impact of that new technology is minimal.

All the technology that defeated the uboats will be in the game by May 1943, and that is good enough to play until 1945, even if you don't get hedgehogs (Which I think actually appeared earlier) and FIDOS.