Sorry if it’s obvious / you’ve already checked it out - have you tried http://developers.webflow.com/ ? I can’t say I have much to do with software development so honestly no idea if that helps!
I looked at that page and I believe I’ve got an idea of how to create an OAuth based app for making api requests on behalf of a user.
However I’m left with no idea about how I can possibly modify the users sites specifically? How can I add new pages, or scripts? actually is that even possible?
You’re welcome buddy! Unfortunately I’ll be of no use to you about your development queries as that’s way outside of my ability, but hopefully someone might see this and be able to jump in. Best of luck dude