[One, Two, Middlesboogie]

Although the definition of 'vestigial' is a now-useless (or nearly useless, in the case of the Glukks; I reckon they can still use their legs for holding things) remnant of something once used a lot (like our vestigial tail vertebrae, nictitating membranes and appendices [which were once caecums]), I don't think that the Glukkons ever walked on their legs. It is highly unlikely that the two pairs of limbs could swap functions like that. If you look at the picture of the octigi family, the Gloktigi and one of the 0.something ones (the small one; not the one supposedly in the mafia) are standing on their front limbs. I reckon the Glukkons used their legs for picking up and manipulating things, and that they had opposable toes as well as opposable thumbs (like a lot of Earthly primates [in fact, I think humans must be the only primates without opposable toes]). But now they use the Sligs do do all their work for them, so their legs have at least atrophied if not become vestigial.
When Molluck has his clothes burned off in AO, he appears to only have one toe on each foot, which would mean that his legs are useless and vestigial (you can't manipulate much with one digit), unless the Glukkons have one large toe and one or two smaller ones (like ostriches, emus and rheas) on each foot that we didn't see.

To answer Kaimana's questions:

so now that we got this far what is the case or reasoning of the M.O.M?

I don't think it's anything less innocuous or more suspicious than an ordinary news network, like CNN or London Tonight.

and what are the historic backgrounds of the three executives Phleg, Dripik, and Aslik?

I've long had a theory that Phleg is not in fact one of the many children of Queen Margaret. He looks totally different from all his peers. His skin is a totally different colour (all the other Glukks are purplish-brown [apart from the Brewmaster, who's greenish because he spends so much time amongst vats of chemicals], but Phleg's more yellowy-chestnut brown), he's far chunkier, wider and more prognathous, and his eyes are a lot closer together. I reckon that there are in fact several queens in each species (although Skillya is the only Slig queen that the Cartel knows about; the others must be in hiding), and Margaret is simply the main, or leading Glukkon queen, and her children get priority for job places in the Cartel. Phleg probably had to get quite a few strings pulled (I like to think that he's also more intelligent than most Cartelian Glukks) to get such a high position in the Magog world.