Again, what can they tap or see into that they couldn't before? All info on the other servers is public, that would be true for any federated server. I really don't get how they'd get any more access to your data than another random person on the internet seeing your profile. They're not making their own instance available to make accounts on, or enable users to post on it directly. You aren't giving them any more details than you would if you had a Twitter account that was public. It is quite literally just for official government information dissemination without being locked behind rate limits.
Surveillance? In what sense, here in particular. A bit confused. Also, it depends on the kind of private instance you mean, since this is private too, in the sense you cannot make accounts on it. What other benefit do they gain over people, using this over a corporate website?
I've only read about it through various posts and discussions on lemmy itself, on their home instance and a couple others. Have yet to understand the ActivityPub model, even after looking at the code for a bit. If I find some clear documentation or post about it, I'll share!
The first two points could be answered better by other people than me, since I'm not super sure on them either. But the third one is basically that. You can subscribe to all other instances' communities regardless of which community you make an account on, so the lemmy model/fediverse itself is the solution/answer to how distributing wouldn't make you lose any data. As long as even one person from your instance is subscribed to a particular community on a particular instance, that content will show up on your instance too, it's basically instances that subscribe to each other. Having bigger instances doesn't really affect on the amount of in-house content as much as the costs of keeping up with it.
It's in a similar vein to having RES for reddit. I never used it without it, so I don't mind it. But yeah, it's something that is being planned to be inbuilt by default so I still don't think you need to make an account elsewhere by any means. It's just appending the post link to your own instance's link. And again, all third party clients already do that by default. I'm on wefwef and it's been a breeze. Till then, if you're browsing on a desktop browser, this userscript does the job - https://greasyfork.org/en/scripts/469273-lemmy-universal-link-switcher The biggest benefit to having it would be that it rewrites the links not just on lemmy, but on other websites that have hyperlinks to it too. So even if you're browsing a website that shares a lem.mee link, it will open up on your own instance instead. Super useful to have not just for using inside of Lemmy, but outside of it too, for a seamless experience.
Nah, you still don't need to open their own link. That's something that's I think is in the works to be streamlined but is doable with browser extensions, when you click on one link you should get to open the post's "local" copy on your instance, which would allow you to post your comment there, and that local copy would sync with the original in a bit so that your comment displays there as well. You should still be able to see the post's local copy if you preface it with your instance's url, but for doing it automatically yeah, third party clients are doing it already I believe and you can do it on your browser via userscripts. Editing to add that if you've subscribed to a particular community and are seeing it's posts there on your feed, you are basically doing just that, seeing it's local copy on your own instance. Which is why you can comment on them without needing to login on to the original community instance.