Have we ruled out simple substitution unwisely?

[oshfdk]

That's fair, although from my personal experience I feel as though people are not encouraged to persue such ideas here. I do often see messages that dismiss the possibility of a simple substitution as well due to entropy issues.

I certainly won't encourage people to spend time on simple substitution ideas and I certainly would dismiss the possibility of a simple substitution, but I'd still have a look. Sometimes even when a solution is obviously wrong, there can be some interesting insights in it.

The caveat is that most "simple substitution" solutions are not simple substitution, but fuzzy substitution, like various scenarios you show in the original post. For example, if

"for every "aiiin" we instead substitute an equally possible set of characters (aiim, anin, amii, aum)"

then this stops being simple substitution already, as far as I can see.