Isn't the reason for the seven day window also to prevent delegating one's HP to several different accounts to be able to self-upvote own posts several times?

I don't think so, as the 7 day post reward window existed before delegation. But I think the delegation "return delay" is based off the 7 day post reward window, to prevent abuse of the type you mention. So it would likely need adjustment if the duration of the post reward window changed.

I agree that the current curation reward system is still not tuned very well. As you've mentioned, it has become obvious that the super-linear reward for early voters hasn't done one of the jobs it was designed for: to reward manual curators versus bots. But it was also designed to discourage "pile-on" voting where everyone just votes on the same trending posts, because later votes give less curation rewards. Again, it's possible it's not really doing this either, hard to say for sure.

But proper tuning is actually quite difficult, IMO, because the intersection of economics and human behavior is often more complicated than people expect. It may turn out that the only way to find something that works well is to keep making adjustments.