Great work ! I want this feature in Sketch too !
I like your switch state solution it’s a clever way to handle the current UI and the split border function. I noticed that in the split mode you cannot add several borders, and it’s logic because in CSS you can have only one border.
So to me, you can easily extend and improve your concept from a “single-border feature” to a “CSS border feature” where you will be able to set the border style from the CSS predefined list. (You just need to remove the global border option panel and add an option panel for each border side).
With this improvement you will be able to get real CSS code like this : border-left: 6px dashed #FF6600;