The resources required by federating depend on how many people follow each other across those two instances and how much these post. Just existing and theoretically federating doesn't need any resources if there's nobody following, assuming threads isn't doing that different from everyone else.

For each post a user makes on your instance, it sends that post to each instance where someone follows the poster. There's no automatic sending to every known instance of every post on your instance.

Yes, absolutely. As an instance owner of four services (lemmy, mastodon, matrix and peertube), the users will absolutely follow a lot more folks (and maybe unfollow them but then the damage is done, metaphorically).

The larger problem will be the habit of having infinitely more people and then meta defederating/limiting the non-meta instances which is what makes them the most money.