How hard is it to fix a fuse with a microscope and a steady hand?

Very hard. FIB is the only known way to do this but even then, that's the type of thing where you start with a pile of SoCs and expect to maybe get lucky with one in a hundred. A FIB machine is also millions of dollars.

You'll need at least an electron microscope... but defeating MCU readout protection using a FIB is actually a thing:

https://www.eag.com/services/engineering/fib-circuit-edit-de...

Costs are what you'd expect for something of this nature.