You've certainly done your homework--to me, the Many Glacier area is the pearl of GNP.
Another, maybe better, way to do the Highline trail is to leave your car at the Swiftcurrent Lodge, and take the Park shuttle to Logan Pass. Then hike to Granite Park, have lunch, and go up to the S'Current lookout, then over S'Current Pass and take the long, beautiful descent down to the S'current Lodge. If you would rather do the Loop instead, then you can blow off S'Current Pass, as there are no views there. Besides, the lookout provides you with perhaps the best view in the Park. You are looking at a long day of hiking either way.
The ranger-led hike to Iceberg Lake is a very slow and pokey hike, since the ranger will stop a lot to show you the flowers, etc. If you are quick hikers, you can take your own hike to Iceberg Lake, and then return to the junction and head up to the Ptarmigan tunnel. That would be a another long day, but it sounds like you are fit enough to do it.
The four valleys in the Many Glacier area (Grinnell glacier, S'Current, Iceberg/Ptarmigan, and the route to Cracker Lake) are some the finest hikes in the Park. Keep in mind that this area also has the highest density of griz, so make sure that the 2 adults each carry pepper spray and know how to use it. Your timing is good though, as the huckleberries probably won't be ripe yet, and the bears will likely be elsewhere finding their meals.
As for the Dawson Pass day, be sure you do that side trip to Upper Two Medicine Lake--it's a most beautiful place. Twin Falls isn't that cool, but since it's right off the main trail, you might as well check it out.
My favorite post-hike tradition in the Many Glacier area is to park at the Many Glacier Hotel's parking lot (above the hotel, just to the east) and take two cold beers to the little rocky knoll between the hotel and parking lot. This is maybe the finest view in the Park that you can drive to. I like to sit against a tree and stare at the valleys of Grinnell, S'Current, and Ice/Ptarmigan, just watching those beautiful mountains erode.
The best reasonably priced dinners can be had at the Swiftcurrent Lodge. In/near Babb, the Cattle Company has pretty good food, as does the Two Sisters restaurant a few miles south toward St. Mary. I like breakfast and lunch at the Babb cafe, probably because my childhood friend owns it (and also the gas station, the motel, and store).
Hope you have some good fun. GNP is a wonderful place.