Okay, some good news, some bad news.
The good news is that I've figured out what is causing the 'jump to other asset string' behaviour that I've been seeing. Here's that second NPC that you talk to, he's the one who expressed shock at your 'robot costume'. Remember it was showing this dialogue, in error:
Here's what he should be saying (and now does!):
The bad news come from the realisation that the dialogue box display codes now appear to be game-area dependent. So I had changed the dialogue box from his Japanese text:
<1a><22>よくできた ぬいぐるみだなお。<pause><1E> え?<pause><1E> ほんもの?<wait>
Where the control codes "1a" "22" generated a 25x1 text box in the middle of the screen.
To my desired english translation:
<1a><70>Man: This robot really\nlooks real!<pause><1E>\nEh?<pause><1E> It IS real!!!<wait>
Where "1a" "70" displayed a 26x3 text box at the top of the screen. Only in this case,
it didn't, it caused the game to jump to another bank and load an asset from the ship-based dialogue (i.e. the bit about a lava planet).
I just tried change the dialogue box codes to "1a" "1e" (24x2 box, top centred), thus:
<1a><1e>Man: This robot really\nlooks real!<pause><1E>\nEh?<pause><1E> It IS real!!!<wait>
...and that works, as seen in the image above.
So, the bad news is that I'll need to go through each dialogue box display, in turn, and work out which ones actually work in the overworld, in town, in the ship, during cinemas, etc and change the english translation to suit. What a goddamn pain.
It isn't the problem affecting the savescreen, that seems to be something else. At least this solves one problem, though generates quite a bit of work to go back through all the asset data and check if I'm using dialogue box codes that don't work in that particular section (and bearing in mind I've not played through the bulk of the game, a lot of that I'm simply not going to know, yet).