1. No, a planeswalker is not a player, so Selkie won't trigger.
2. Yes. The fact that you control Kataki doesn't change the fact that the trigger's source is the artifact itself, not Kataki. Since your opponent controls all the triggers, he gets to stack them in the order he likes. He can stack them to achieve what you suggest. I covered issues like this in example 6 of
this article.
3. Not a rules question. This has been discussed at length elsewhere.
4. Not a rules question. This has been discussed at length elsewhere.
5. Yes. One way to to do it: In response to the comes into play trigger you just bounce the Sculler (i.e., return it to your hand using Chain of Vapor or something), causing its leaves play trigger to trigger. Since the leaves play trigger is now on top of the stack, it resolves first, but does nothing since a card hasn't been exiled yet. The comes into play trigger resolves and then a card is exiled. At this point there is no way for the exiled card to come back because the trigger that would return it has already resolved.
6. You can't interrupt the resolution of a spell. If you have a more specific question, please ask it.
7. Yes, this is possible.
8. Not a rules question. This has also been discussed at length.
I think you would benefit greatly from reading my article,
Priority and the Stack. Note that the combat rules have slightly changed since this article was originally published, so it's a little outdated in that regard.