Which rail pass, if any?

I feel as though the Eurail Global pass may be a bit overkill?

You got that right. It only gives you a 25% discount Days 9 to 13.
You only have the 3 miles from Chiasso, Switzerland to Como, Italy on the train. It does not work on Italian buses or Como lake boats.

You might just find that the Swiss Half Fare Card will save you the most money. For an STP, an 8-day Swiss Flex Pass would probably be the best fit, costing 472 CHF for 2nd class.

From the Swiss border rail station at Chiasso, you can take a local bus or taxi to Cernobbio (or walk 5.5 km). You probably won't be on a train within Italy at all, so there's no need to consider passes that cover it. Any local transport you use around Lake Como should be purchased there as you go.

Thank you, Sam!

Would it be beneficial to purchase a 4-day Berner Oberland regional pass and a 4-day STP? or would they ineffectively overlap? The added benefits of the Berner Oberland pass are appealing to me, based on what we plan to do in that area (Cable cars, boat tours, etc).

Thank you, Laura. I thought that would be the case.

Oberland Pass and STP overlap and would be just wasted money for you. STP covers everything all over Switzerland while regional pass is just for the Oberland.

A slight difference is that the B.O. Pass fully covers the Wengeralpbahn from Wengen to Kleinescheidegg to Grindelwald, Lifts at Maennlichen, and First, Schynige Platte, and a few others. STP covers some of these at 25%, some at 50%.

You would be paying 323 CHF for a 4-day STP plus 260 CHF for the 4-day BOP. Now you have shelled out 583 CHF vs 472 CHF for the 8-day pass. Will you save 111 CHF by doing this? Maybe, maybe not, especially if the weather is not ideal.

As some have said, rail passes are all about convenience, not saving money. You'll spend less by buying a Half Fare Card for 120 CHF, but you will have to buy a ticket for each journey.