Alright looks like I'm gonna have to elaborate.
My only concern with this is that the point of the set was to be able to break typical Fairy checks employed by bulky offenses and balances. If you run Toxic over Earth Power you can't touch Klefki and certain niche Steels. You also now have fewer options against Mega Gengar. Some overlap is created since the set already handles Pokemon such as Ho-Oh, Giratina-O, and Arceus-Dragon. The primary gain you get with Toxic on this set are Lugia and Arceus-Water, but they can Toxic stall you just as well as you can do to them (you must hit them on switch-in or accept the trade if it is beneficial). I think at most it could be slashed after Earth Power, but a moves mention might also be the appropriate choice here.
As for the Arceus-Ground set it's probably fine to write up. I'm not sure why you think it can't be used on a Ho-Oh balance when the bulk of what it checks is very comparable to dual attacking Arceus-Water. You can allocate Defog to some Levitating Dragon and have plenty of ways to build defenses against Darkrai and EKiller. These arrangements also have room to fit a Water resist which is a concern since you're starting with a basis of P Don / Arceus-Ground / Ho-Oh. I think there are a number of traits that make this set not totally outclassed by Arceus-Water.
As for the toxic slash I seriously think earth power Arceus-Fairy has no chance of beating any steel outside Klefki anyway. Having options vs Mega Gengar is good but the fact is that its best set runs no poison move... You still 3hko Gar in practice which is just fine. The gain of toxic isn't that you get to toxic walls like Lugia (altho getting the tox on Arceus-Water is part of the move's niche), it is that you can prevent some offensive mons from sweeping you. Random CM Arceus, Ekiller etc. I don't think Arceus-Fairy should ever be run on a balance due to there being better options for those teams, so having toxic as a fail safe for the aforementioned CM Arcs (which offensive teams generally have more trouble covering, even they are super rare) is important. The only team I managed to make with the mon had toxic > epower and I would never recommend anyone to change that. Another big point in toxic's favor is that you get to drop status on most defogers, such as Arceus-Water, which is something that I feel is super important for offensive teams. Think of it as Arceus-Fighting in BW2.
Regarding Arceus-Ground on Ho-oh teams:
Ho-oh teams are balanced by nature and thus often a bulky Arceus to fill a role of a physically defensive mon. Arceus-Ground seems like it would work but I don't think you quite understand its niche if you point out teammates such as "levitating dragons". That's what people sometimes do now and I think it's a big no no and a complete bastardization of whatever positive traits special groundceus. The fact that you can allocate 1 slot on a team to check/counter any variant of Pdon is something unique about the set and whenever you resort to pairing it with additional Pdon checks such as "levitating dragons" the result is an overall weaker build. Special Arceus-Ground is infact always outclassed by Water if you resort to pairing it with a mon like Latios, Giratina-O, or Latias. So yeah, I think the mon is overall better suiting for more offensive teams, with teammates like Rayquaza, Gengar etc.
So the defog point is covered, but I also think it's important to address the Ho-oh weakness of Ho-oh balanced builds. The reason these teams almost always run Arceus-Water/Dragon as their physically defensive backbone is due to additional fire resists. This is largely due to Ho-oh balances being forced into using Klefki/other steels for a Xerneas check (Ho-oh+Pdon is never enough). I wouldn't be able to figure out how to fit special Arceus-Ground on a Ho-oh balance due to this predicament... Additionally a Kyogre weakness will surely happen. Running something like Pdon/Groundceus/Ferro/Ho-oh/Diancie/defoger?? might be your best bet to not be overly Kyogre/Ho-oh weak (it still is Ho-oh weak though) and finding a suitable defoger that doesn't make using Arceus-Ground moot seems impossible to me.
Overall I can't see how this mons would ever fit on a convential Ho-oh balance, but on a more offensive team that relies more on priority (Ray, Deo-A) for Xerneas I think it puts in work. The counterargument is why I'd run this over SD Arceus-Ground I guess, but I believe the Mence checking attribute comes in handy as well for those teams.
I really wasn't too intrigued with my first look on this set. It really just looks like a waterceus clone that deals with pdon better but gives up the ability to deal with birds and levitators. I think it would be kind of difficult to build a team around this since it needs defog support as well as mons to help alleviate the bird weakness. But I do think the fact that it's able to check every pdon set and mmence both is a really great trait that not many pokemon are able to do, which ups it's value in my mind. I'm still not too keen on the set though and personally I would want to see a build where this set works well and doesn't hold it back by just being a waterceus look-a-like.
I agree with Hack on the toxic slash on fairyceus though.
You just said exactly what it does better than Arc-Water. Then you jump to the conclusion that 99 % of players do- "I need defog". I wasn't really expecting people to be stuck in that mindset but I guess it's natural. I view the set as a role compressor for offense in need for a strong sole Pdon check and passable Mence answer.
some teams:
Pdon/Deo-A/Gar/Ray/Xern/Arceus-Ground
Cloyster/Ho-oh/Pdon/Gar/Xern/Arceus-Ground