I watched both renderings of Bluebell Polka (both well done, in my ears) and did not see any "faking". These melodeons seem to have extra keys for C and F at the top.

Will, have you considered writing simple harmony voices for the diatonics, only using the available notes? (It would not be "faking" as described by John, since other instruments would play the correct melody.) Even if Bb and F are missing, you'd still have the D to represent the Bb chord. For the chord accompaniment, the capoed guitars will do.

You would be in good company, for example with Joseph Haydn writing for natural horns and trumpets outside their native keys in his symphonies. (BTW this is also a good way to form a large orchestra at family celebrations, including countless kids playing up to three different notes on their borrowed recorders etc., while the professionals jam.)