YY,
In the glossary of ESPB Vol 5 Child states 'swords bent in the middle'
'Nonsense or near nonsense'. He was helped with his glossary by William Walker and a whole swathe of Scottish antiquarians. What this means is none of these antiquarians had any idea what it meant.

Your second version, as I'm sure Child would have told you if he were alive, is most likely Peter Buchan's attempt at rationalising the nonsense, but it still makes little sense. According to Child by the way Burns was resposible for the original nonsense. I gather he liked a drop or two, and we know he had a sense of humour. I can picture him sat on his cloud rubbing his hands with glee at our attempts to make sense out of his nonsense.

You'd be just as well going back to 17thc broadsides and writing your own version like the people above did (IMHO). Neither of these two verses occur in the other versions given by Child.

If you really want to stretch the meanings as given, if you'd had a few drinks you might describe a claymore as being bent in the middle!