There may also be a play on words here, in that 'borrow' can also (in Scottish usage, at any rate) mean "to pledge ... in liquor"; that may be implied in the first verse you quote.

"But I am come tae wield my brand" means (in Standard English) "But I am come to wield my brand". The usage is quite old fashioned, and nowadays mostly confined to poetic diction. 'Sword' is a perfectly good substitute if you feel the need to change it.

'Wield' is still used; not very much in everyday conversation, but extremely often in fantasy novels. There is an implication of skill and control that isn't well served by substituting 'swing;' I should stick to 'wield' if I were you.