Using Secp wasn't an option. For censorship resistance, size really matters, and Mainline DHT is orders of magnitude bigger than the next bigger overlay network and I don't think humanity will ever bootstrap a DHT in the millions of nodes ever again.

Mainline (as almost every other overlay network) uses Ed25519 exclusively for mutable items.

As for Nostr events, I assume you mean as the format for the data, and here again we went with DNS packets, since you will be hard pressed to come up with something more appropriate for the use case and the encoding is brilliant, definitely better than JSON :)