#1) Sorry, can't help you here, not nearly enough experience playing/running the game. (If you got an open spot, PM me. I would love to play some HARP 😁 ) I would say, for ease, just have four options: Full OB, Half OB, Quarter OB, Full DB (i.e., no OB).
#2) Looks like you only need a high open ended roll.
#3) Similar to #1, but I think it would be best if you could make it so they can add more.
#4) Don't tie it to the grid, you can still use the measuring tool with the grid off, in Roll20; I'm not sure about Foundry. I don't know how to move without snapping to the grid on Foundry, but it might be similar to Roll20 where you just hold the alt button while moving. If so, you just measure the distance and then move without snapping to grid.
Hope that was of some use to you.