I just came to what seems to me a reasonable conclusion:

The four and twenty came FROM Strathdon to take Eppie back with them to Strathdon. She's not saying she'll not marry, but none of the raiders or anyone else from Strathdon is acceptable.

Can anyone shed light on the geography of this song? "Carron", I've interpreted as perhaps a river forming the boundary between Strathdon and whatever Eppie's area name is. Does this make sense?

And as to the imputation of impotence, this could be the meaning, but not as a fact, only as a further insult to one who couldn't "carry the day", so to speak.

Dave Oesterreich