if playing darius legend. and you have 1 energy. you have a hidden kennen keeper of balance at a battlefield. you flip it, tap legend. then use that to pay the 2 to stun. is this allowed? does this order work? can you use kennen to trigger legion on legend to then pay for that same kennen?

Yes — this works, and the order is fine. Legion on Hand of Noxus checks whether a card other than the one with the Legion ability has been finalized this turn (Rule 812.1.c), and Darius is a Legend that's never played, so Kennen being finalized satisfies it. Kennen's "When you play me" trigger uses the resolve meaning of play, so by the time it asks for the [2], Kennen has already been finalized and Legion is active.

You can then exhaust Darius mid-resolution: activated abilities that Add resources and have Reaction may be activated any time a spell or ability requires resources be paid, and they finalize and resolve immediately, even during another effect's resolution (Rules 429.3, 444.2.c). So your 1 floating energy + Darius's 1 = the [2], and the stun happens (same pattern as paying Hard Bargain's [2] with Darius).

Two caveats: tapping Darius before playing Kennen does nothing — with no card played that turn, Legion isn't met and no energy is added. And since Kennen was played from Hidden, the "when you play me" stun must target a unit at the battlefield where he was hidden.