The whole purpose of this arc was to depict Rudeus's depression after he got left behind by Eris, so aside from his growth as a combat mage, which has been happening, he obviously couldn't have really good time. Considering what kind of story this is, it's not so surprising ED would be introduced as a part of Rudeus's problems. To show that, you pretty much needed Sara or something equivalent. I'd say Rudeus badmouthing her wasn't so peculiar either. That's his personality, in the end. It didn't help Sara, after their cancelled night together, told him that she never really cared about him that much and the whole thing was just to pay him back for all the help. That, if anything, should have reminded Rudeus of how Eris broke up with him. So, while still somewhat affected by all the booze, Rudeus let that grudge out of his system.

I could have lived without this arc, for sure. The new characters don't really matter. All of this could have been covered in a single ep, if done wisely, through dialogue and strongly displaying Rudeus's depression visually. I trust showing Elinalise is a sign of the end, especially since we got "future" talk about Rudeus's achievements.