I quite like the idea of a single strap from the right hand end and round your right shoulder only, though, it's the only arrangement suggested that avoids complicating the bellows movement. Easy enough to put on too.
I tried this at home - it slips off my shoulders. However, One strap going from concertina's left end over the left shoulder, crossing the back and under right urmpit into and underneath right concertina end - superb results! Easy to put in, on and off, centers Concertina's right end in the middle of stomack(belly, spare tire, 6 pack), leaves bellows free from rubing and bellows movements are not complicated at all. Instrument is super secure, steady.
I'll make some photos of this too. My camera battery was down.
That sounds good, a lot like the way you run a guitar strap if I'm reading this correctly.
You are reading this correctly.