Thanks again to the several friends who responded to my FB appeal for “a large suitcase you don’t need to see for a year.” The one I ended up borrowing has the distinction of being large enough to fit me inside it, while staying about an inch under the total maximum dimensions for SAS checked baggage (29+21+11 = 61″, max allowed = 62″). The trick, therefore, is to keep it also within the weight guidelines (50 lbs max, of which the suitcase itself takes about 16).
Aaaaaand the verdict is: nope, not unless Mills comes to the rescue by adopting heavy objects into his checked backpack. Which happily, he does! And all our assembled bags — the humongo-suitcase, Mills’s modest pack extra-stuffed with heavy-ish things (while the giant suitcase goes at least a third empty), one knapsack apiece, a 5-string fiddle, and a nyckelharpa — fit handily into the little Insight.
The last remaining transport hurdle was to get both instruments on board safely without challenge. Mills carried the fiddle. I printed out a copy of this handy article describing the relevant FAA ruling of Feb 2012, in case of emergency. But when it came to it, we just gave ourselves pre-boarding status by joining the line a bit before our row was called, and didn’t even get a raised eyebrow in response. Whew!