Have we ruled out simple substitution unwisely?

[eggyk]

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.

I was suggesting this to see its affect on entropy, not as a decoding attempt. 

In such a case, the theoretically correct solution would be a simple solution, or not a complicated solution at least,  but we just wouldn't be able to tell apart which symbol is which. 

Again, if we had the word "minimum", written frustratingly like iiiiiiiiiiiiiii, if you know which lines are which it's simple (i guess its not even substitution), and if you don't know, its not.