The mechanic who told you it is still the fuel pump is wrong. The problem is caused by a power wire between the fuel pump relay and the fuel pump, which is shorted to ground. It may be a chafed wire somewhere; or may be a wire in a harness which has melted from another short. The relay in the trunk you tested is not the fuel pump relay; it is an anti rollover fuel cutoff relay. The fuel pump relay is in the engine compartment or the passenger compartment. But don't bother with those because bad relays do not normally cause fuses to blow. You need to test the wiring. Sometimes the fuel pump power wire may be touching a hot exhaust pipe; or it may rub on the driveshaft. Those are places where shorts to ground can be created.
Please do not run the fuel pump when it is outside of the tank. These pumps MUST be lubricated by fuel when they are running. Even running a pump for a few minutes without fuel in it can ruin the pump. And then you'll have a second problem on top of the original one.
By the way; you can buy a high quality new fuel pump for this car from www.rockauto.com
for less than $34. They have 3 different pumps for this application, at prices ranging from $27.79 to $33.99, plus shipping.
But getting back to sorting out the problem; try turning the key on, with a new fuse installed; when the fuel pump is not connected. If the fuse does not blow; the problem is most likely a ground in the connecting wires that attach to the pump inside the tank. Sometimes these wires will be pinched by the tank flange seal as they pass into the tank. These are the places to closely inspect.
You can also temporarily run a new wire all the way from the fuse box to the fuel pump in the tank. This is a sure way to rule out any problems in the wiring harness.