Putting a stronger fuse is always a bad idea. The fuse is supposed to be the weakest part of the circuit. Now, perhaps the wiring is the weakest point, which could lead to burned wires.

 

Yeah, MAYBE try a bigger fuse for diagnostic purposes if you don’t mind risking frying some wires (glad that worked out), but NEVER leave it in.

So far you’ve been lucky. Stop driving it until you get the correct fuel pump and fuse. And get a new mechanic.