I can Valkyrie away from Ori's combo.
Valkyri-ing away from an Orianna combo doesn't mean you escaped, because it's targettable, it only means you went deeper into your death, unless it's a very specific situation where you run across a creep wave while escaping. That's not more reliable than "place ball" -> profit.
I have to Flash away from Diana's.
The ball can't be stunned, slowed, peeled off or anything else, so once again, your argument falls flat. No, you don't need to flash, you only need to stun or slow her and walk away, then kill her because she is melee and peeling make her useless. Whereas for Orianna, it's no big deal, she will just reposition her ball again in less than 3 seconds.
Additionally, the cooldown on that combo is significantly better for Diana as well.
No, she doesn't, because the combo is not something Diana can do freely without consequences, like Orianna does when she repositions her ball. Diana either goes in and everything goes well or she goes in and dies. You either do it correctly and at the right time or you get punished for it. Orianna is not punished for it, because even if she blows her R for nothing, the safety of her range means that nothing was lost and she will casually go back to farming her lane in order to get that 300 cs and be able to insta-gib 3 people with a couple spells.
Calling Diana more reliable when Diana has more ways to be stopped available for the other team is an overstatement at best, if not slightly of a joke.
The number of times a champion has been nerfed is irrelevant. The history really doesn't matter. If Caitlyn got nerfed by 8 AD one patch, versus being nerfed by 1 AD eight patches in a row, the end result is the same. I'd urge you to not include things like that in the future: It clutters your message.
Also, don't go pulling deductive fallacies in order to excuse, you are very much aware that ALL of Diana's nerfs were considerable and she was left an empty shell with no valuable assets to bring to a team.
You're acting as if all of those nerfs were justified, even though they clearly weren't and went overboard, yet there is no sign of willingness to do anything about it.
Release Diana, sure, broken as hell, no objections there.
2nd wave of nerfs, fine, let's slide along with it, you just destroyed her assassin identity, let's say she wasn't supposed to snowball that hard. Not everyone can have everything they want.
But in no way can you justify the nerfs after that point, where you also destroyed bruiser Diana, with sunfire cape/abyssal scep..
You could say you don't want her to be an assassin,fine. But if afterwards you guys come out and say that you don't want her to be a bruiser either, something is wrong with you.
Oh, and something else:
Base stats vs. passives, siphoning power, etc.: It's a gameplay thing. We're not trying to say, "Yo guys, Diana is THE attack speed champion!" We're not saying, "Lissandra is THE manaless mana champion."
Why not? Didn't you guys say that you want your champions to have defined strengths and weaknesses? Wasn't that the reason given behind avoiding nerfing Lee Sin for so long?
How do you outplay RWEQ from Diana?** It literally can't miss**, and she casts it from out of range of anyone's basic attack.
Not sure if serious ...I'll just leave this here then in case you are though. 
In short, you said reliability is a big asset. But saying that Diana is more reliable is kind of a hollow argument, because she isn't when you take into account how easy it is to stop her and peel her off. You're only taking 1v1s into consideration and not actual teamplay, which is supposed to be the core of the game.
Edit: Don't take parts of the post personally, I know you ain't directly responsible for balancing, it's just being part of the company, you know how it goes.