Checked my wiring diagram for a '84 928S.
Your FP relay has a Red/Green wire coming off terminal #87. This wire is the suspected short/open wire as it goes right to the pump to supply battery voltage. It is also fused, fuse # 13 on you fuse box, make sure it's still good, you could have popped it when you jumped out the relays earlier. Wire color can vary but generally this is the color Porsche used for that circuit, in any event it's the same wire that comes off #87 on the FP relay.
If the fuel pump relay 'clicks' when you jumped it out between 30 and 87, check for power at the Red/Grn wire (at the relay #87). You should have power. If not the relay is bad. I think we already established that it is good, correct? Make sure though.
If the relay clicks and you have power on #87 at the relay -but not on the pump- then you must replace the Red/Grn wire.
Remember, the ful pump itself has two wires, a Brown ground wire that goes right from the pump to a good chassis ground, and the power wire that goes from the Fuel Pump Relay #87 to the pump.
Use your multimeter to check the Red/Grn power wire for short/open. If it's bad then cut the old wire(leave some wire on either end to splice) and run a new wire directly from relay (#87)to the fuel pump. While you are at it replace the ground wire(you said it was melted at insulation) Run a new ground wire to a good clean spot on the chassis.
OR an alternate method...
1)- Disconnect battery
2)- Make sure fuel pump ground wire(Brown) is good and not damaged, if not replace this wire first.
Run new wire directly from fuel pump right to chassis, wherever you can find a good clean spot. You mentioned that it was damaged earlier so just replace it now to prevent another short.
3)- Take a long jumper wire and connect it between FP Relay #87 and the power side of the pump (Red/Grn wire).
4)- Don't jump out the relay, just from relay #87 to pump power.
5)-Reconnect battery
6)- Check fuse #13.
7)- turn ignition on and pump should work now, but keep in mind that many fuel pumps only get power with ignition on -not cranking- for only a few seconds.
If the pump comes on now -and it better- then you just need to re-do step #3 using a new wire instead of a jumper wire.
Good Luck, keep us posted.