We have the mechanism for handling that in monsters.txt: rather than saying the HP is 99999, record it as [99999] - anything in [] is an expression. We use that for scaling monsters, but a constant expression works too.
I expect so. Are you volunteering?
The first point was a slightly off-topic "bug" (i.e. monsters.txt would need updating, presumably after someone else confirmed this).
For the second point, I should have a multi sitting around in aftercore; not sure if it's the right sign, but I can ascend if not