An interesting thing to consider, whether society would be better driven by forced cooperation or by greed as it is now. I would guess that, facing the threat of earth becoming a frozen wasteland, humans would continue to hate and fight because we are stubbornly stupid like that. @Nightweaver20xx
The castle was converted into a public library with rooms for visitors to the school of friendship.
I think there are two tiers of alicorns: ascended alicorns like Celestia, Luna, and Twilight in the epilogue and regular alicorns like pre-coronation Twilight, Cadance, and Flurry Heart. What the difference is (besides flowing hair) or how that power is gained/transferred is a matter for headcanon.