I *think* I've now sorted the save-file issues.
After hours of stress testing, which showed up oodles of issues, I've inevitably decided that I should make the values Unsigned, since in 99% of my games, the scores tend to go up, not down.

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It seems that with the switch to Unsigned ints, longs and even bytes (chars), the save files are much more stable.
There's been a heck of a lot of complicated, and somewhat messy tweaking today.
But I've made everything that much more stable.
It's also much less tweakable, so I'll have to watch if I decide to add any extra stuff into the saves.

Now I'm going to have to faff about with the MacOS version, and figure out where I can safely save scores on there, because it apparently doesn't like me dumping data inside the "Application"'s folder.
D'oh!