a) When you're using Discord, IRC, or a forum, all the conversations happen in separate gardens, and you have to jump between them. Mastodon lets you look into each of those gardens, see who you like, follow them, and talk to them, all in one place. There are some great communities, but you don't have to pick one of them. You use them as starting points to make your follow list. And it's not just Mastodon, it's also the Fediverse, which has Diaspora (like Facebook), Lemmy (like reddit), personal blog software, spinoffs of Mastodon and the others, etc. And you can connect to all of them. So it's like if Discord, IRC, and whatever else also worked together.

b) If their your friend, you know their user@domain, because they tell you, same as they would their email address. Unless you're on an Antifa instance and your friend is on a Nazi instance, it is highly unlikely they will be disconnected from each other. If you join one of the instances on joinmastodon.org they have agreed to give 3 months notice for any shutdown. You can move to a different instance at any time, export your follow list and redirect to the new instance.

c) They're already joining and there's a list on fedified.com . Of your use cases, this is IMO the least important. Most of the celebrities will join Bluesky and that's fine. But not only does Mastodon allow you to verify using your personal website, fedified gives a list of important verified accounts, and a number of organization specific Mastodon instances have started up for people in those organizations.

Mastodon is not Twitter, it isn't supposed to be. But if the point is to have social media with interesting conversations with friends and people you'd like to discover, it does that. And it can't be destroyed by a single billionaire. If the point is an algorithm that makes you sad and sells you things, and brands that want to grow followings to sell things, either hope Twitter recovers or Bluesky works.