I've been reading but...
I think Malcolm has got it wrong with his assessment of Noteworthy.
I can't eplain NWC having problems converting to a MIDI but MIDI to abc is always likely to cause problems. The root of the problem is probably best explained by anahata here.
I don't know the Notworthy format but I suspect it would, like abc be geared around musical notation rather than musical performance and as such, I would guess (it's outside my programming experience) reliable conversions between the 2 formats should be quite possible.
The abc/nwc conversion routine some of us use is 3rd party product btw.