Thanks for the inspiration for this idea, by the way :) I might not have thought of it if not for Luminosity and Radiance.
And, speaking of which, something I was wondering about: Is your name actually inspired by the alicorns from MLP? Believe it or not, I only thought of the association a few weeks ago, but I wasn’t curious enough to PM you about it.
I’d say you’ve got two out of three there. Based on lines Chrysalis says (When she beats Celestia, she says “Ah! Shining Armor’s love for you is even stronger than I thought! Consuming it has made me even more powerful than Celestia!”), her power doesn’t depend on the magical strength of the pony she’s feeding off, it’s all about love, and alicorns don’t necessarily love any more intensely than other ponies. The changelings would have been more powerful taking on alicorn forms, but it’s clear that that isn’t enough to win in one-on-one combat: The Mane 6 took out dozens on their own, and they weren’t even warriors. Twilight was the only pony who actually had the ability to do a lot of damage in a fight, and arguably Pinkie. Shining loved Cadence much more than most ponies love other ponies, which gave Chrysalis the power to beat Celestia. When the two fought, Celestia was actually the one to attack first, and it was a fairly straight contest of strength which Celestia lost. You’re right about the uncounterable nature of the love spell that defeated the changeling invasion though.
As for Discord and Sombra, you have good points. Especially with Discord. It seems fairly obvious that Discord is stronger than Celestia, as Celestia is powerless to stop him in Season 2, but at the same time, when Celestia cast a spell on the Elements, Discord didn’t even TRY to do anything to overcome the protections of the one thing in Equestria that could stop him. As you said, that’s a pretty clear indication that he knew it would be a waste of his time to try.
I’m assuming Sombra had some sort of power source: My best guess is that he fed off the negative emotions of his subjects, which caused a feedback loop where he became more evil and more powerful and caused more misery which looped until he was strong enough to fight the alicorns and win without the Elements of Harmony backing them up. Said power source would in fact be FAR more dangerous in the hands of an alicorn, even the version of an alicorn I intend to use in my story, where alicorn-ness is basically a force multiplier for magic, rather than an instant pass to godhood.
As for the precognition: If Celestia could see the future, the fic would turn out extremely differently. In fact, the entire fic would probably only last one or two chapters. As soon as Twilight finds out Celestia can see the future, her likely response would be to use that. She could make up her mind to do X, where X is a series of factors that could influence how the alicornified society would develop. If none of them work out positively, game over. As you said, you can’t really argue with someone who sees the future with tremendous reliability. If one of them does turn out favourable, Celestia has no leg to stand on, and logically would have to give in. In fact, if it turns out that, for the fic to make logical sense, Celestia must have sufficient precognition to make this a possibility, I simply won’t write it.
There are other potential reasons behind the first episode turning out as it did. Chief among them is that Twilight and the others THOUGHT they had to save Equestria, but in reality, Celestia was waiting to intervene. She wanted Twilight and the others to succeed, because the Elements would be stronger and have a better chance of working if they did, but if they were clearly outmatched, Celestia would have stepped in herself. (Even this is a risk, but I don’t find it too difficult to believe Celestia would take some level of risk to get her sister back to her pre-NMM state.)
So, to summarise my stance:
Binary magic: Agree. Celestia is comparable to FAI: DIsagree.