Aaaaaaaaaaaaaaarg! Where is your head at?

I don't understand. How is calling a byte string a character string useful?

It's not "calling a byte string a character string". It's calling a string containing multi-byte characters, a character string.

Because the only way SvPVX() can return a pointer to more than 2 bytes of ram at the C-level, is if the SV contains a scalar containing 2 (multi-byte) characters at the Perl level.

If the scalar contains a string containing just two bytes, then SvPVX() will return a pointer to just exactly 2 bytes. It can do nothing else, as we've at length established, because SvPVX() performs no coercions.

(You still haven't demonstrated how you can get those four to exist with only two string types)

You know that +1 and the assigned \0 in your code, don't allocate +1 and don't put \0 and there you go.

Now you're either just taking the piss, or you're talking gibberish.

Because there is no +1 in my code, and the only assignment of \0 is into the output buffer. Which has nothing whatsoever to do with the input accessed via SvPVX().

The "those four to exist" to which I refer, are the four possibilities that you claimed SvPVX() could return a pointer to, variously, either 2, 3, 4 or 5 bytes of ram.