Diana has worse base damages and ratios than Orianna in every single of her spells.
Diana's E is literally the utility portion of Orianna's ultimate and Dissonance. Only that she doesn't have the luxury of being able to do it from a safe 825 distance that Orianna's Q allows. The slow also lasts 1 second less and the knock-in effect is a bit weaker.
Just as food for thought, devil's advocate, and whatever:
Reliability is actually a really big part of that. Orianna Q-R'ing you versus Diana R-E'ing are very different, and Diana's is much more reliable. I can Valkyrie away from Ori's combo. I have to Flash away from Diana's. Additionally, the cooldown on that combo is significantly better for Diana as well.
To address random other points you put in:
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.
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."
Rather, we're saying, "Hey, Diana should feel like she attacks pretty fast all the time. Her passive should always feel useful." So rather than say, "Okay, you start pretty fast, and then go SUPERSPEED" we flattened it instead and say, "You start as literally the fastest-attacking champion in the game, and get a little bit faster in the end, pretty close to everyone else too." Now you could make the argument that Diana should be a more AS-focused champion, but taking out base stats and placing an equivalent in an ability is not about cheekily nerfing and buffing a champion, it's about crafting a specialized experience.