Is it ever possible to use "if yes" and "if no" instead of "if so" and "if not"?
Can I say "to subsitute [something] with [something]"
if so/if not is much more common. In the context of a form with a Yes/No answer if yes/if no is possible.
You do indeed substitute actual_something with ersatz_something. Alternatively you can substitute ersatz_something for actual_something. 'Same difference' [they're equivalent].