1. Harrier target selection and frequency
Quinn's AA is pretty weak early-game, I feel like even with full AD & creep-kill masteries she has trouble last hitting compared to most ADC's- Harrier target selection (the animation) could be a tad quicker and the creep-target choice happen closer to creep-death (5%-10% closer IMO).
2. Burst potential during Tag Team
Burst during Tag Team is fine, one thing I'd like to see is a CD refresh for Q+W+E when you R, it ruins the "opportune moment" to jump in & strike if you've just Q'd & then a team fight breaks and now you're stuck without for Valor Form...
Single-target burst or tower burst is too good IMO: lvl 11 with fair AD, pop R & watch the towers melt, that AS steroid too powerful.
I have a proposed change: change Valor passive attack-component to something like Gangplank's Passive.
In line with above proposed change, I'd additionally propose a change to the W passive & it's interaction with Tag Team... cut the AS steroid in half, but add an equal MS steroid component (a la built-in Furor action). And then, during Tag Team that passive could be "on (active)" for the duration.
These changes would eliminate Tag Team's tower powning potential (which' too great), synergize well with her catch & kill motif, and help her kite/peel which needs it IMO (vault isn't a very reliable escape in the least).
Also, I think doing this- you could slightly improve her base AD & scaling AD.
3. Chasing sustained dps as Quinn (not as Valor)
It's okay, see above proposed change (would help you to sustain damage as you wouldn't feel like you need FM or BoTRK to catch anyone quicker than you and keep damage going when R was on CD)... I think upping her Attack Range by 25 would make it PERFECT.
4. Did you successfully use Vault to escape over a wall?
Yeah I have, I feel like it should go a little bit further, it's very difficult to pull off if you don't have all the time in the world (intense situation, nigh impossible, esp if something interacts with the creeps you want to vault over like enemy aoe)... with above proposed change (+25 Attack Range), Vault range could also improve +25, effectively solving this.
PLEASE NOTE THERE IS INTENSIVE DISCUSSION GOING ON IN THIS THREAD REGARDING POTENTIAL PITFALLS OF THE CHARACTER THAT I BELIEVE NEED ADDRESSING~ PLEASE TAKE THE TIME TO READ THROUGH IF YOU MAY, <3EYRGOS.