Ok. I'd started to answer before you posted your 'over-explanation'
. You've got a lot going on, so let's take it in stages. I understand why you are wanting to use .php extensions in order to differentiate between your existing .html pages and your migrated pages, but it sounds like your existing site is not using Modx. If that is so, the 'Modx' version will be hidden until the cutover and so you can leave the pages with their current identifiers ie potatoes.html.
By setting up furls, you are changing from this: mysite.com/index.php?id=2 to this: mysite.com/potatoes.html. The id=2 part refers to the resource id. In order to turn this into 'potatoes', you need to edit the resource - specifically the resource alias field and add 'potato'. Once you have done that, and cleared your cache, you should be able to access the page as mysite.com/potato.html - which you don't want.
To remove the "html" extension, go to Content/ Content Types and edit the HTML content type (by blanking out ".html"). Again, after clearing the cache, you will be able to access the page as mysite.com/potato.
If you really
want mysite.com/potato.php, then I suggest you change the resource alias from potato to potato.php. However, I have no idea whether Modx will like this.
Hopefully that also answers your question about Content creators, although I think still think it is a bad idea to have to append .php to all your aliases in perpetuity.