But the Rasengan that was used was just a Kyuubi enhanced Rasengan, it had no Fuuton properties like RS since Rasengan is just shape manipulation on a high level.
I honestly believe that Enton was just a plot device for the two to get their arms blown off. I don't see how Enton would have helped Sasuke in that situation. I feel like he should have attempted a bigger Chidori too. Then it would have been a stalemate but with their arms most likely intact.
Ushing a stronger chidori would of resulted witht he same outcome the resengan is a superior justu its the perfect justu to counter other chakra based justu
I believe the amaterasu did serve a purpose
Sasuek states that its useless as a straight up attack as showcased earlier its not fast enough to target naruto and his cloak would just stop it ,as sasuke said its best used as a diversion.
So he placed it inside his chidori so when his chidori struck the flames would them consume him the chidori was a means to come into contact the flames were meant to end it.
But as you pointed out the resengan is just chakra and all it needs to properly defend oneself against other justu is to increase its outage