"lend an ear' is used, I'm sure, simply because it's long been, if not in common usage, at least in common knowledge. Even if someone started out singing 'give an ear", they would in all likelihood end up singing "lend an ear", making the change unconsciously.

"Doctrines" would be all the rationale and argument behind and in support of the Jacobite cause.

I think it's impossible now to pin down exactly what each phrase in the rest of the song means, but I would say you've got the gist of it, which is all you can hope for. Burns' audience probably would have got more precise meaning than is available to us.