Wow! I think you may have fixed it enough that it's workable. The funky blocks are still there, but the crashes don't seem to be occurring like they were.
I really can't thank you enough - my daughter has put a ton of work in on this. May I ask what editor you used so that I can try using that in case the issue happens again?
As I said - I was hoping to avoid re-converting the world as it existed at the time Better Together was released, as this will result in a lot of lost work (about a years worth). If that is the only suggestion or support that can be offered, then that's what I'll have to do.
I have not, We converted the world from Minecraft for XB1 to Bedrock-Edition very shortly after Bedrock edition was released with Better-Together (which was almost a year ago), so a lot of work has been done in the world between then and now.
No - there was apparently only one block missing the texture. At least - I hadn't discovered any others that looked out of place.