It may be just a matter of semantic but opening an m file is typically done in order to edit the file. On the other hand, code typically run m file, which is a different operation.
It is trivial to decide which m code to run based on user input.
It's not very clear what you want to do with the second part of your question. If you're asking that the code then change actual values that are hardcoded in the m file, then you need to rethink your design and use functions with inputs.
