Remember, Siddhartha Gautama Buddha, the most famous (in the eyes of us outsiders), but far from the only enlightened turned back and didn't enter nirvana before he started to teach.

Huh? The Buddha reached full enlightenment before forming the sangha, according to tradition. You can make an argument that he spent many years afterwards developing the path (in Theravada terms, that he reached stream-entry but not arhatship before teaching, for example), but this has nothing to do with his teaching itself. Or maybe you mean parinirvana, but this is necessarily the last thing anyone does.

(You are of course right that bodhisattvas have a surface similarity to immortal player characters.)