The fact that he couldn't awake it in his old age and knew how it works is only you assumption, many times characters awake powers and they use them right away without having any previous knowledge on how to do it. A prime example of that is Obito himself who after became Juubi's jinchuuriki knew how to use all of its attacks. What Madara said about how to unlock it, is that he was about to die when he did and that's exactly what I used as an argument. The sage might have awakened without becoming a jinchuuriki only if we consider the fruit his mother ate as a product of Juubi's chakra and therefore if you eat it, in sage's case he got that chakra through his mother, he could have awakened it at first as a pseudo jinchuuriki and then after defeating it and sealing it becoming a perfect jin.
Off topic, you do realize that you self-contradict yourself here right? Previously you said that he wouldn't be able to master it at his old age and yet you supprt that he could do it in the middle of a battle.
In any case, if the sage did not have it from birth and if you're against about what I said about the fruit being a product of Juubi's and therefore having its chakra, then explain me how a person with powerful chakra only had two sons who had so distinctive characteristics which are so similar to the ones Juubi has? ( I don't think that I need to show you scans to prove it). It's not possible only because Juubi= senju + uchiha the equation of rinnegan= senju + uchiha makes sense.
Even if Naruto momentarily gives up then he still can awae it after fusing the beasts' chakra inside of him.
Lastly, ofc all this thread is about its possibilties, as you said there are too many gaps to cover it by saying something is definite. Even if I had all the necessary clues, I wouldn't be in position to say what will happen with certainty. A theory no matter how good it is remains a theory. However, that doesn't mean that my theory doesn't hold any value. I compared all the people who had rinnegan till now and took from each one a specific clue which helped to establish a theory based on the similarities between of them and by explaining some possible scenario based on logical arguments which can be supported from the conclusions I derived from this comparion. I just wanted to show that not everthing is a clear as people and to make them think that it had never been said that the sage had rinnegan from birth, yet all people believed so and that when Obito became jin and didn't awake it, everyone said that Juubi's jin= rinnegan is BS and yet no one has ever said that by becoming one you awake it immediately.