"coming after a right-repeat at the end of the previous line, is also syntactically incorrect - there's a bar there with no notes in it, which BarFly would correctly flag as an error"

Repeated section often begins and end mid-bar, so there's plenty of precedent to suggest that a repeat mark isn't necessarily a barline.

It seems quite clear to me that two different conventions (broadly speaking, folk and classical) are in common use, and good ABC player or MIDI converter would allow the user to choose which interpretation to apply.

Anahata