Nothing moves, screen goes black and crashes as soon as end turn is hit.
I think that's good news Suggests it may not be a province/border problem, as the AI factions are still "thinking" and nothing has encountered any borders yet.

The early file is a mess anyway, so I may end up rewriting the whole thing on a new one based on the high file, and then tidy up the layout.
Rather than do a total "new build" of the file, I would go the "conversion by parts" route. Start with your functional file, then copy across various sections and test each version till you get a crasher. It all helps to narrow down the search - of course, the problem might not be in the latest part you added, it may just react with it, but at this stage you need anything that can narrow the field...

Have I? I've as yet done nothing with the minimap and the campaign selection map as I prefer to base them on the main map once it's finalised. Could this then be the problem? Though if so why does my modified map work with the vanilla early.txt file? AFAIK you can add any provinces to the map so long as they are in the palette. If you don't declare them in the startpos file they will simply be disabled, with no other adverse effects. That's how I had always understood it anyway and is how my lukupmap2 seems to work when placed in the vanilla game (tested that also).
You have - the FE_MINILUKUP is the overlay for the "campaign selection map" so once you're in a camapign its job is done You can do all sorts of horrible things to the minimap, so long as it's a BIF and the right size. I have had versions so screwed up that only one province out of 57 showed up (and that in the wrong place ) but the game still worked fine... and again it would affect all eras... so that's ruled out as the cause, I believe.

But all that said, if it is just a startpos typo, the question then is how come it loads in the first place and gets as far as it does ?

It does sound like you've tested all the likely bits, and from what you say I think your LUKUP.LBM must be fine as well. Hmmm.... I have to congratulate you, Caravel - it is a good one