Yes, that was what I was referring to. However, this is assuming that Lemmy has properly implemented webfinger and doesn’t store direct links (which I haven’t checked).

Alternatively, you could proxy all requests with application/activity+json in the Accept or Content-type headers.

You can, but other servers will not recognize it as the same server.

There are however ways to run your server on a subdomain and make it appear as if it’s still the original domain. Is that what you’re looking for?