Yet more additional stuff!

Alicorns can grant other beings Sponsored Magic (as in the Dresden Files RPG). They don't actually know that, though.

It was Cadance's idea to send the letters back to Twilight in Return of Harmony Part 2. They knew that Twilight had fallen to Discord's manipulations, and Cadance knew the letters returning would be enough of a kick to snap her out of it. All three alicorns apologised to Spike afterwards, though.

On Twilight becoming Star Alicorn: yeah, that's in my headcanon too, like I've said before.

Does anyone know if there are any good fics which involve the rest of the Mane 6 becoming Alicorns as well?