What is one's relationship with the world upon attaining Nirvana (and how do we work to get there)
From my own experience, at certain point in one's practice, one's ties with the regular world reduce so much that one no longer gets any energy from participating in mundane affairs ("There is nothing further for the sake of this world").

At this point one has only two choices: either go 100% hardcore ascetic ("Arhat") or engage with society in the teaching/helping capacity ("Bodhisattva") - either openly or secretly. One can't really be "normal" anymore, pursuing success and entertainment, since one's value system no longer goes up to worldly goals. So if one were to engage with the world lukewarmely on samsaric terms - then one would either die miserable death, or regress badly -- hence only two reasonable choices. I suppose this is why it is often said that once you go far enough there is no turning back.

As to which option is preferable, I think it's totally up to personal inclinations. Some people are very self-sufficient and very comfortable being the end in themselves. For them the peace of minimalism is way more agreeable than the high drama of Bodhisattva's path. Other people need to get energy from the outside, so they chose to give something, to get something in return.