You seem to recognize this is a bit of a pickle, and I don’t think there’s a great answer exactly. Ideally, I’d call the airline and see if you can switch to flights to Catania, or even an open jaw Catania-Palermo. It’s worth trying, and on the sliding scale of money-time-convenience could be worth paying an extra fee for. You never know. I’ve had good luck once you get a human customer service person on the line.
If you can’t? Eh, I might take the L and keep myself on the west side of the island, include Trapani/Erice, Agrigento and Cefalu. Even that is a ton for six days. If you’re driving, you could stick with Palermo and include a day or two for Etna activities, but that’s still not much time. Same with Siracusa, ideally you want a few full days there, especially as now you’d be expending a lot of effort (and time) to get there. For me, the math just doesn’t compute. This is why folks usually say you need a couple weeks for Sicily. This west to east travel complicates matters. Perhaps your Etna agenda has to wait for another day for you to really give it its due.
