“But it’s hard to call it a Middleman site anymore” Not sure I agree, if its built on top of Middleman, I use Middleman to contain a pattern library more than anything please see - https://codeblender.net/
If it contains Mini SPA, Widgets, JS Callbacks then fine, its still static in the fact that there is no Server side calls to simply render views.
Of course to do anything with Auth and Commerce requires server based knowledge and languages - that would be the same with or without middleman.
I think my initial reply answered the question that you can build anything with Middleman, but as they said they had limited web development experience getting into the nitty gritty was mute, I think learning the basics of Middleman would be a start.
You could interface with say Facebook Open Auth also - without server side too… persistence would be the issue but session based login is fine with middleman.
Please read - https://jamstack.org/