Thanks Jim, Mike and the rest... this is one of the knottiest transport puzzles we've ever encountered. I'm starting to think our whole trip plan is just a bad idea, but I haven't given up just yet. Delta (which is combined with NW) out of Sault St. Marie, MI will take only ONE bike per flight, at that outrageous $175 fee. Leaving one behind doesn't seem like a very good option! The other small airports (Pellston, Traverse City, Alpena) seem to use the same model small plane, so don't seem any more likely to take 2 bikes.
Tomorrow I'm going to phone all the bike rental places on Mackinac Island (6 as far as I can tell), and see what they know about boxing and shipping bikes from the island. Then just getting the two of us and our panniers to an airport and home should be relatively simple. Other option is to get us and the bikes to Pellston, rent a car or van and drive to Detroit or back to Milwaukee.