I personally think your only failing was in explaining the situation to the band : no melodeon player = no band = no gig. If the booker is then happy to accept an alternative performer for the same slot, that's not an issue with the band. Would they perform as a duo without you?

I play with a four-piece, members are over 100 miles apart. From the outset we have arranged and rehearsed material we normally play with any combination of two or three of us, or which two of the members perform solo, in addition to four-piece numbers. This means that if anyone cannot make it, or has done a flounce, we still have a gig and rehearsed material. Gives the audient some variety and each of us some time at the bar and an opportunity to tweak levels on the PA.