smallpiper,

you're generally right that folklorists and historians (and organologists, musicologists, etc) are biased toward a theory of a single origin for an idea followed by spreading of the idea abroad, rather than a whole bunch of origins. This favors and simplifies their methodology of tracing ideas back to their origin. It is one of those instances where, if you aren't careful, your theory can shape your evidence rather than the other way round. But I think in the case of bagpipes in Europe, the evidence also happens to support this theory. The lines of development from a one-droned medieval bagpipe to most modern bagpipes can be followed through pictorial references and surviving examples. So most historians would be comfortable saying that most European bagpipes--including the ones in Britain and in Ireland--were derived from this model. However, before that, as we've seen, things get murky, and certainly the basic idea of adding a bag to a reed-pipe may have occurred more than once.