Here's what Planewalker has to reconcile the 3e teleporting fiends with the loss of that power in Hellbound's 'Squaring the Circle'. Keep in mind this was my opinion at the time and it seems to have been accepted by consensus. However since we havn't formally written it up and posted this in one of the releases yet it's obviously subject to change.
"However my take on it is different... The existance of the Maeldur Et Kavurik is clearly something that the Yugoloths had absolutely no intention of leaking to the other fiends or any of the other planar races. By forcing any deals with the Baatezu or the Tanar'ri they tacitly admit its existance, and open themselves up to suspiscions of their larger role in the Blood War than just self serving mercenaries.
They would be admitting many things they had no intention of admitting to if they deal with the fiends to restore their teleportation abilities.
Rather this is what I would see them doing: Take the Maeldur back to Gehenna or the Waste and hide it in the darkest, most secure hole on the plane, metaphorically speaking. Secret it away. Feed to the Maeldur the names of the other fiends as fast as possible, scattering it out among the other fiends at random to make it appear a gradual and natural process. Some quirk of the planes essentially by which it was removed in the first place.
Start spreading stories about how many of their own have lost the same abilities and are only now slowly starting to regain them. Start killing those berks who would say otherwise, and attempt to hunt down and kill any living Baatezu involved in the raid that captured the Maeldur in the first place, as well as all those Baatezu who crewed the Relentless.
Give themselves plausable deniability and in secret restore the other fiends' teleportation abilities.
By dealing with the other fiends, they essentially are forced into playing their hand far far earlier than I could ever see them playing it. It's not something that honestly should have been meant to be used as a tool to control the other fiends until the Yugoloths intended to stop the Blood War. That isn't going to happen till many other of the plans of the Baernaloths come to fruition, or their experiments reach a conclusion. For one, the 3 great Yugoloth towers are not linked yet. The 3rd tower, The Tower of Incarnate Pain in Carceri is nowhere near finished, and likely won't be for another few thousand years. After that who knows what other plans the 'loths as a race have in store towards their promotion of evil across the multiverse.
They likely would eventually desire to exterminate the Gehreleths, and thats not even begun. Likely not till they finished the 3rd 'loth tower. And thats just two long term goals they'd likely need to finish before attempting to end the Blood War and unite the fiends under their own thumb.
Dealing with the fiends to restore their teleportation powers is simply too blatant, too obvious, and too rash for the 'loths to do. They're subtle creatures, and they need to act that way to ensure success. They're not ready yet to admit to what they are entirely, not ready to drop that veil across their nature they wear so well."