This one has been fixed:
>>> f"{'a\\'b'}"
"a'b"

This one was a bug that I previously fixed, that Martin pointed out:
>>> f"{x!s!s}"
  File "<stdin>", line 1
SyntaxError: invalid character following conversion character

And this is the same bug:
>>> f"{x!s{y}}"
  File "<stdin>", line 1
SyntaxError: invalid character following conversion character

I'm wrapping up my new code plus tests. I'll post it Real Soon Now.

Thanks for your help.