The second thing you requested is here, to redirect a community to your instance.
But posts, I don’t think that’s possible, at least I tried a lot and met a few impossibilities due to the fact that posts are just numbers in the url like this: localinstance.tld/post/numbers, and these numbers will be different in remoteinstance.tld/post/differentnumbers.
User flairs would be cool. Also a script that opens the current instance’s version of a post or community when I click on an external instance link.
I think since it is a useful feature the developers of Lemmy are or will work in it with no need of scripts
The second thing you requested is here, to redirect a community to your instance.
But posts, I don’t think that’s possible, at least I tried a lot and met a few impossibilities due to the fact that posts are just numbers in the url like this:
localinstance.tld/post/numbers, and these numbers will be different inremoteinstance.tld/post/differentnumbers.https://sh.itjust.works/post/70143