No it isn't. My whole argumentation is built around Heroes IV Death Campaign and Might and Magic IX.
I'm quoting this just to make sure that you won't edit your post later and claim something else after you have finally finished the first 20% of the HoMM and M&M games which are relevant to the current discussion and which you haven't even bothered to check. Moving on:
If they have to go to so great lengths to create a volcano to summon them up, they must have grown awfully dependent on them, which given that those creatures are able to travel about freely asks the question of why weren't they at Colony Zog?
Because they are not Kreegans? Why don't you live in an ant-hill?
If they are clearly acting seperately, what is the functional difference between your "allies from the plane of fire" and my "Gate-Kreegans" other than that the former is a sentence and the latter a word?
The only creatures in the Inferno line-up that are clearly from the plane of fire are the Efreeti, which are NOT Kreegans. If you want evidence - read Xyron's description in HoMM IV, from what I remember it addresses the Kreegan - Efreeti alliance unambiguously. The Gogs seem to be native to Antagarich, as you can find them roaming areas well outside Eeofol in M&M VII. It could appear that even the Demons and the Pit Fiends aren't Kreegans, but some of the "real" demons on the planet - see HoMM Chronicles - Conquest of the Underworld. The only creatures in Inferno which are 100% Kreegan are the Devils - this is the Kreegans' "nickname" and they die in a fashion very similar to the devils from M&M VI (just to use your own reasoning again). So yes, all these allies are quite valuable and again - no, they are not Kreegans. Questions?
Whether the Kreegans wish it or not, turning the world into a fiery wasteland is the ultimate result of Kreegan domination
Whether the modern society wants it or not, turning the world into a polluted pigpen is the ultimate result of its domination. Therefore the modern society is actually a society of four-legged mammals that we know as pigs. Therefore I have just proven that we are all pigs.
Creating a volcano cannot be an easy feat (else we'd be able to use it to destroy annoying well-garrisoned fortified cities in the game).
Where did you read that in the game? How do you know how easy is it to create a volcano and what is the actual process of its creation? What happens if these volcanoes can be "created" only where you have pockets of magma relatively close to the surface? Or if the respective spell requires days or even weeks to be cast? Or... I hope you get the idea.
It's simple enough. Logically the Kreegans must eat more or less the same things as other biological creatures or else their multiplication and expansion would be no threat to those other creatures.
Sorry, this is pure nonsense and it contradicts even your own arguments. The Kreegans consume the resources of any planet they conquer, leaving it barren wasteland. What you are saying is that if some extraterrestrials now land on Earth, destroy all natural resources and leave, this will not be disastrous for us at all and even on the contrary - we will still have enough spare time to have pointless discussions on the Internet forums.
Again - we have no idea what do the Kreegans eat. And if we go back a little, the idea that they come to destroy, not to consume, is your own, so go argue with yourself about it.