It’s not just lemmy that’s benefiting from Elon Musk.
Is there a need to have a lemmy and Mastodon?
Theoretically they’re somewhat interoperable but if you plan on using both you’ll probably have a better experience having an account on an instance of each service.
My understanding is that isn’t correct - in Lemmy you currently can follow Mastodon communities, but not users (following users on any fediverse platform including Lemmy itself just isn’t a part of Lemmy yet). I believe it’s planned to be implemented, but this is one thing differentiating Lemmy and Kbin - you can currently follow Mastodon users in Kbin, and in that case I think it’s just the same way you’d follow another Kbin user. To find their user page it would just be kbin.social/u/user@their.mastodon.instance instead of kbin.social/u/user, so I assume it would be similar for Lemmy once implemented
