Going back to Aragorn's oath to Frodo, you could possibly say that he hadn't broken it in that he rode out to the Morannon outnumbered, to help Frodo. Granted, other motivations were included in this, but certainally Frodo's well-being was one of them. I don't see that letting Frodo go off on his own neccessarily means that he has broken an oath. He didn't afterall, swear to stick to him like Sam.
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