Okay Arceus-Normal is excellent, and there is no denying that. However, it's often overestimated by newer players, and there are many viable ways of getting around it. Also, most well-prepared teams have a surefire way around it and backup checks as well, ranging from Terrakion, Forretress, Giratina-O, to Ferrothorn, Arceus-Ghost,Groudon, etc. What I'm trying to say is that the metagame is pretty well adapted to Arceus-Normal, and that it has flaws that hold it back, being its complete lack of defensive synergy, its opportunity cost, and relatively weak initial offensive presence. Furthermore, as soon as you reveal your Arceus-Normal, if you get stopped the first time, your opponent is certainly not going to give you the chance to set up Swords Dance a second time. Burns also hinder Arceus-Normal a lot more than they'll hinder Arceus-Ghost.
1. Arceus-Ghost usually has the bulk (Calm Mind + Recover + EV investment) to work around Choice Scarf users, especially late-game. It can also tank an attack and respond with Judgment, which is much more powerful than ExtremeSpeed. If Arceus-Normal can't KO with ExtremeSpeed, it's probably getting KO'd in return. Choice Scarf Terrakion, Kabutops under rain, and maybe a few other can outspeed, tank ExtremeSpeed, and hit back hard.
2. Ho-oh is Arceus-Ghost's one big counter, and saying that Arceus-Ghost can't get past its counter is pretty obvious. Arceus-Normal is obviously not sweeping if the opponent has Groudon, Giratina, etc. It should be noted that Arceus-Ghost at +1 has a chance to KO tank Ho-oh after Stealth Rock (+1 252 SpA Spooky Plate Arceus-Ghost Judgment (Ghost) vs. 248 HP / 84 SpD Ho-Oh: 180-213 (43.37 - 51.32%) -- 18.75% chance to OHKO after Stealth Rock).
3. Arceus-Normal has a 80 BP STAB move, which is only slightly less pathetic. However, Calm Mind Arceus-Ghost (which in my opinion is much more effective) has a 100 BP STAB move. Swords Dance Arceus-Ghost is pretty bad, though can be surprising. I don't know why you said the listed Pokemon were obscure; basically all of them are common sights. ???
4. Your scenario works for basically any good sweeper. A lot of Pokemon at +2 can deal heavy damage to teams, especially if you give them screen support. Arceus in general takes hits better than most other sweepers, but Arceus-Ghost is superior to Arceus-Normal in this regard as it has a superior defensive typing. Arceus-Normal has no resistances, while Arceus-Ghost is immune to Fighting, resists Bug, etc. and is a very durable spinblocker in a hazards-centric metagame.
5. I don't really understand how this is relevant. It's been stated that Rayquaza is a wallbreaker and Arceus-Normal is a sweeper. Okay. However, I think what superstar was trying to say is that both get access to ExtremeSpeed, and Arceus-Normal isn't alone in having +2 priority. If a player wants to place Arceus-Normal for another Arceus forme that can serve as a better glue for the team, Rayquaza can provide priority as well. Speaking of glues, Arceus-Normal is a standalone sweeper that doesn't provide anything in terms of defensive synergy, and the opportunity cost is important (at the cost of say Arceus-Grass on a team weak to rain, Arceus-Steel on a team weak to Dragon-types, etc.).
Personally, I'd have Arceus-Ghost in S+ and Arceus-Normal in S-, but since we're not doing + / - for S-tier, than I'm fine where things currently are.