If Hydration were broken on multiple Pokémon and were more widespread, I could see a case for a complex ban. But I don't see how you can justify doing for Manaphy only, because of the precedent that sets.
It can be justified because otherwise, a Pokemon is being banned despite not being broken. At least, assuming it is indeed found that Manaphy is found to not be broken outside of Drizzle. I'm not saying this necessarily
should be assumed, but we need to find out if it's the case.
And yes, if Manaphy is not broken outside of Drizzle, Manaphy is not broken. There is one core suggestion made by Aldaron's proposal that allowed him to create an exception for Ability + Ability bans. That suggestion is that permanent weather changes the game so much that even if a Pokemon is broken under a certain permitted weather condition, that Pokemon is not necessarily broken overall. That applied to Kingdra, Ludicolo, and others, and now it applies to Manaphy.
Now, if this is taken as justification for Ability + Ability bans, it has the unfortunate implication of having certain Pokemon be banned from being used in certain conditions, even when those Pokemon are not broken in any conditions. Because of this, a Hydration + Drizzle ban may not be the way to go, but there is an alternative. Aldaron's proposal created an exception that can only be applied to permanent weather, but there is no reason that abilities are the only things that can be banned under permanent weather. Because of this, I'm changing my stance. What should be banned is not Hydration + Drizzle, but Manaphy + Drizzle. This can also be applied to the Swift Swim + Drizzle ban. Aldaron's proposal has both short term and long term implications. The short term implications are to test whether or not Drizzle is broken without Swift Swim. Assuming it is concluded that this is the case, then we can take it as a conclusion that Drizzle does not need to be banned and start tuning down the scope of the ban. Rather than banning all Swift Swim abusers under permanent rain, we should decide which Swift Swim abusers are broken under permanent rain and ban those specific Pokemon from being used alongside Drizzle.
This all falls under the precise exception made by Aldaron's proposal, and none of it is any more complicated than the current Swift Swim + Drizzle ban made under that proposal. Now, what
would be more complicated would be to ban those Swift Swim abusers only from using Swift Swim under permanent rain. I know that this was considered, and I am not opposed to this, but if it does not happen, it would mean that Pokemon such as Kingdra and Ludicolo would be banned from permanent rain even with their other abilities. I don't know what extent of negative implications this might have, but it certainly seems as if it might have negative implications for Ludicolo. That is the one argument I can see being made against this, and it will have to be considered. But consideration of this is what I am suggesting, not immediate acceptance.
On the other hand, is allowing the non-broken versions of Pokemon such as Ludicolo in the distinct conditions in which their Swift Swim versions even what we want? Even under Drizzle, Swift Swim itself is not broken, because there are Pokemon that are not broken even with Swift Swim. And banning specific abilities on specific Pokemon was shot down, so it isn't, to use it as an example again, Swift Swim Ludicolo that's broken under Drizzle. It's Ludicolo that's broken under Drizzle, and according to our current policy, Ludicolo should always be banned under Drizzle. This implies that reverting to our original policy of specific Pokemon bans really is what fits our current policy best. The only difference is that these specific Pokemon bans only apply in combination with specific permanent weather - which, based on Aldaron's proposal, is acceptable.