Whether you use the direct solder screw terminals or the pluggable connectors won't make a difference in whether loose wire strands or something shorts between wires. The biggest difference you'd be making with the heat bed relay is that any high current issue related to the heat bed is kept on the heat bed relay, not the main controller. As mentioned earlier, adding an inline fuse would be one way to ensure some protection beyond what the power supply itself will do.

Whether you move the heat bed control to D9 sort of depends on whether you're ever going to want to have a print cooler controlled by the slicer with that controller. We don't even know that the D8 is unusable - the picture isn't good enough to offer much input on that. I'd pull the RAMPS board off the Arduino board and inspect both sides of the board any trace or component damage. If you don't see any, reinstall it and test it, maybe using a light bulb or voltmeter as an indicator rather than the heat bed. If that works I'd then wire in the heat bed just for the shorter heat up time.

Since the 10-inch i3v comes with a heat bed relay by default, this must be an 8-inch i3v?