However I strongly disagree that its all a plan by the baernaloths and that they originally created the Tannari and Baatezu.
That's cool, the myth is partly for my campaign, where the Baerns are playing the long game. They actually helped the Ancient Baatorians create Asmodeus, and by extension the Baarezu (a story for another time), and did the same with the Obyriths and Tanar'ri. So they didn't create all the fiends personally, but they had a hand in it. All it really goes to show is that I've read an unhealthy amount of Shemmy's Storyhour. The baerns do exist canonically though, there is one in the final mod of Hellbound, "Squaring the Circle." That's a minor point though, back to the hoardlings!
Drake
What I'm saying is that the 'loths would definitely be aware of the consequences of occupying a lawful plane, and because of this they'd have some way of dealing with it because they don't want to be tainted by law or chaos. (if they can make Ghenna spontaneously generate mezzoloths, it's a fair bet they can also modify the plane further in a way to make it more to thier liking).
Alternatively, they may want to occupy Carceri as well to ofset the lawful taint.