Is that for real? I've often played Orianna vs LB, and though I don't think she is invincible, I tried a few times to position myself to aim my Q at her when I know she will attack, and use E to hit her on the way back, but I don't think it's ever worked, she appears "to me anyway" to blink passed the slow moving ball, avoiding both the outgoing and incoming damage. Ahri's collision area is better, and though I have not seen the match, it would be reasonable to assume that LB was hit in brief periods inbetween, not "while dashing" whereas oriannas attack is slow so it's easier to completely avoid by "blinking" passed it in one direction, and passed it on the return.
As I said, i could be wrong, just wanted to check, because I gave up on the tactic, but if it "Should" work maybe I just didn't notice the damage or did something else wrong. these days if I know she used W on a minion wave, I place the ball on her return pad and hit her with AA's. If you position right, like I said, you should be able to shield yourself, causing her to take damage by the returning ball, and if you're right, she should take damage if she returns to her pad or stays put. But like i've said, it's never worked before, so I just W her when she returns to the pad.
On a side note, I agree with the OP, a reduced mana cost for using W a second time sounds fair, in theory, equally, a much bigger nerf would be a drain on mana while she is away from her W pad, so the longer she stays in her second location, the more mana she loses. That has a lot of negatives on her, while the OP is more subtle