I was going to say I can use any Pokemon whatsoever because I don't need them to cover anything else, but after I went out there and tried it (with Deoxys-A + Darkrai, minimum defensive synergy lol), I was wrong. One slot in the team is essentially free - I can use Deoxys-A and get away with it, which means I can cover every threat in the metagame to some extent with 5 Pokemon - and can be used for anything, but any beyond that and I need to be very careful. But with only that one spot I can't spare the Arceus forme (I've found that Dragon Arceus is an essential part of my remaining 5 Pokemon, although it *might* be replaceable with Latias or Grass Arceus).
I don't like the Swift Swimmers because they all aren't very strong. In a land of 150 base SpA, Kingdra's base 95 SpA is ludicrously weak, Swift Swim or no. Don't know about Omastar. My experience with Shell Smashers is that you had better sweep with them completely (as in once you Shell Smash, you end game), or be able to Baton Pass out. Omastar doesn't look capable of the first without some heavy team support, and it doesn't learn Baton Pass, so yeah.
Are there any underrated or effective-but-uncommon non-Arceus sweepers out there that I can use? I know there are a lot of Arceus formes I've never seen before, e.g. Ice / Electric / Bug / Fire Arceus, but off the top of my head I've seen everything else (including for example Choice Band Arceus @_@).
In my opinion Arceus is an excellent Pokemon. It has only a few outstanding sets (the Extremekiller is one of those) but its versatility is unmatched. Dragon Arceus is on my team solely because of its resists, its bulk, and Recover - I needed a sturdy Water, Fire and Electric resist and Dragon Arceus was there for me. If I need something that can absorb Thunders but still pose a reasonable threat, I can use Ground Arceus; if I need something to absorb Toxic Spikes, I can use Poison Arceus. It is possible to build an effective team without Arceus, but since Arceus is so slippery, it can easily slip into any other Pokemon's role and do it well e.g. if there were no Arceus, I would probably be using Latias, but as it is I can't really see why Latias is not outclassed bar some strange set (Tailwind, Healing Wish, etc) by Dragon Arceus. It's not that Dragon Arceus forces itself into reckoning by its sheer power, but rather that it just does things better than Latias.
Btw about Xatu, I've faced it a few times but like Gastrodon / Dugtrio / Jynx it seems very niche to me. Sure when it works it works great, but otherwise it's simply a big liability. It doesn't even stop entry hazards reliably: you can stop Forretress (Volt Switch hurts) and Ferrothorn (Gyro Ball hurts), but trying to stop Dialga or Groudon from putting up SR is suicidal. You wrote the analysis; how did it work out for you?