I'm A Banana
Potassium-Rich
I don't think that avoiding all conctradiction is possible (it may be possible in many cases, of course, but not in all). For instance, I don't see how you can say without contradiction that (i) mezzodaemons are a type of demons, and (ii) they are the same thing as mezzoloths, and (iii) mezzoloths are not a type of demon but rather are members of a race of demon progenitors/breeders (I'm a bit hazy on the Planescape-y details).
I kind of look at it this way: if a mezzoloth is with a cadre of demons, working to support their goal of ultimate destruction, how does one tell the difference between that mezzoloth and the other demons?
Historically, it's mostly been alignment, and thus, to a certain degree, behavior. In 3e, this also meant demons were vulnerable to certain spells that the mezzoloths weren't. In 2e this was to a certain degree true as well, as yugoloth had slightly different immunities/resistances.
So, if immunities and resistances are simplified, and perhaps yugoloths in general get some meta-fictional twist so that there's some CE corruption among them, the difference between a CE mezzoloth and those demon allies becomes....essentially the same as telling the difference between a vrock and a marilith.
Those yugoloths that have "gone native" are, for all intents and purposes, demons. People call them demons. They work with demons. They may even be pledging allegiance to demon lords.
And in games that really love the 4e lore, this is all the mezzoloths. The difference between the 'loth and the demons is effectively nil in this scenario.
With the idea of maximal lore, this just isn't considered to be all the mezzoloths. These mezzodemons can exist alongside other mezzoloths -- the mezzoloths from pre-4e -- just fine. We use some sort of justification for the fact that some change (perhaps there's a demonic parasite that forces one to act CE; perhaps the 'loths have perfected magic that allows them to change the very fabric of their being to adapt to new planes and allies; perhaps some 'loths have thrown in their lot with the creatures that hire them, etc.), and run with it. It leads to interesting new places.
And for the people who want to excise non-4e lore, they just apply the idea that SOME mezzoloths have become mezzodemons to ALL.
And for those who want to excise 4e lore, they just apply the idea that SOME mezzoloths have become mezzodeomons to NONE.
And for those who just want to use good lore, maybe the parasite that turns you into a demon gets loose in Podunk, and the PC's find themselves working alongside an Aranoloth that has an interest in learning how to counter-act this demonic plague infecting the race.