d8 d9 d10 are interchangeable, one is not normally for the bed. The edits are in several places in the firmware, the easiest is in config.h. just change to a different ramps configuration.
Why would you get a new board, you have 3 ports you only messed up 1, if you are worried you can just replace the mosfet. make sure they are not touching each other.
Yes just a box for the electronics so they can’t get metal chips on them or touched. Make sure it is vented and preferably with a fan, this generates a lot of heat.