Arceus is going to have a barrier for entry (collect all the mains) and the one chance as every Mythical does but I doubt there will be a second chance if you fail (Unless you use saves but who would do that :) ). Arceus does have a reginal human partner as well so I doubt it will be a Pokemon that simply comes back later in a different update. As for the actual content of the capture, I wouldn't know but your idea of it being a mix of most of the mechanics used so far feels fitting for scale and I'm going to assume was Chainsaws plan. Just know the next update is split and Arceus is being added in the second part.