Super!
So my wife and I will plan on taking ~3 days to explore the area.

We've decided to skip Milan, (spend more time in Florence).

Plan would be Florence to Milan, then transfer at Milan and head toward Tirano...spending the night along the way or in Tirano itself.
(I think that the Rhaetische Bahn is in this area, so we would incorporate it here :-0

Next day leave Tirano, go to St. Moritz, Davos, Chur...staying somewhere along the way.

Next day wake up, go from Chur to Zurich and catch our 7pm flight to London.

Does this seem reasonable?

I'm going to skip the car as my wife gets terribly car sick (with my driving ^ ).