A large amount of the things you mention become less dangerous in the event of greater alicorn presence in Equestria, not more. Nightmare Moon, Discord and Chrysalis ALL almost won, and if even just a few dozen alicorns had existed, they wouldn't have stood a chance in hell.

Now, the whole existential risk angle...is a very interesting point, since based on what I've just argued, the logical meeting-ground between the two would be to have a task force of alicorns, say, at least a dozen, but no more than a hundred, all comprised of ponies Celestia trusted sufficiently. The chance of alicorn-related existential risk increases, but the chance of the next season's villain killing everyone plummets to nearly zero. So, given your predictions on the power of alicorns, you're right. If we take the prior that alicorns automatically gain Celestia-level powers, it's far, far too dangerous to give everyone that kind of power, and immortalising everyone is a very, very bad idea.

In fact, this very argument leads me to believe that, in order to provide the optimum amount of conflict in the story, alicorns need to be a lot more powerful than unicorns, but not automatically god-tier. Alicorns should have the potential to reach the power of Celestia and Luna, but imagine if Celestia and Luna were immortal unicorns: Based on their great amount of knowledge, they would still likely be more powerful mages than any other unicorn alive. So this could easily extend to alicorns as well. This still brings about the existential risk angle. Powerful mortals can cast spells like Want-It-Need-It and the altering of Parasprites already, but a lot more ponies would be capable of such things if they were alicornified. My own personal belief about alicorn power levels subscribes to this idea, but as another LWer pointed out, I shouldn't make the world convenient for me. I should make it as inconvenient as possible while still allowing the protagonists to win, because that makes for a much, much better story than "Deathists are always wrong about everything forever." But I don't think this is a problem that allows rational protagonists to win. They'd have to back down.

As for your FAI question: The answer is, no, I don't want to convince a Friendly AI of this, but Celestia is not a friendly AI. She's immortal, she's the ruler of Equestria, and she's definitely much wiser than just about any mortal, but she's not a superintelligence. She's not so far beyond ponies in mental ability that the concept of challenging her judgement is ludicrous. She has pony-level intelligence, just a lot more years to learn things. But, as we can extrapolate from elderly humans, sometimes age has it's deficits as well, making people more inflexible in their opinions. Your argument for existential risk is what would convince me if I were Twilight, not Celestia saying it's too dangerous and me blindly trusting said judgement. Celestia knows more than Twilight, but not so much more that in an argument between the two, Celestia can never be wrong.