Desperate for urgent help: How to fix Macbook Pro when repair, erase, reinstall do not work

Slow performance can be an indication of a hardware problem. such as a failing hard disk, or some other problem.

Have complete and current backups. That's your path back to your data. Old backups are better than no backups, but they're far from preferable. But backups that aren't available aren't really very viable backups, unfortunately.

As for performing hardware repairs locally, the folks at ifixit often have teardown and repair information posted. This if you or somebody else on the island will be performing a repair, and this assumes the problem is failing storage hardware; there can be many other causes for the symptoms reported. You'll want to try using Target Disk Mode with a compatible system and a cable, otherwise you'll have to first remove and then copy the disk using external tools.

You'll also want a local copy of macOS to install onto the system, preferably created on a USB device or other bootable storage available locally. Some of the MacBook models are old enough to have a DVD drive, and — assuming that DVD drive is working, as not all are — macOS bootable DVD media can be acquired and booted. This macOS software will be needed for installing onto a new (blank) disk, if that's what you end up swapping.