Can we add MI?

I'd love to include more states (especially MI, MN, and TX), but to avoid having too many branches in the tree, I had to draw a line somewhere. FiveThirtyEight, for example, has MI as 92% likely for Biden, but NH at 87%. I will keep an eye on the forecasts, though!

Is there a way to make an RD like this accept integers like you do above with the swing states, and then illuminate the path towards a sum?

Yes, that sounds like it's a similar use case to what I did here. Each swing state corresponds to an integer which is the number of electoral votes.