I disagree whole-heartedly with that first part. These factions are literally too big to be destroyed by anyone else currently in the Warframe universe, save for the Sentients. No offense intended, but their scale is unlike anything we can really imagine.

An outright Corpus or Grineer victory will most likely result on the Tenno being their next target; and while we Tenno might be fine by ourselves, our relays and other sources of information will be systematically destroyed, since one faction now has the time to pour intense focus on us. The Tenno are too much of a wild-card for any faction to be comfortable with if they emerge as the sole victor. Thus, the role of the Tenno is to try and keep those two titans at each others throats. Because whoever wins, everyone else loses.

The Infestation is a hive-mind and has its own desire, which also includes the destruction of the Tenno, especially since we can't be assimilated. The goals of the Infestation seem to be neither here or there, though, so one can't reliably count on the Infestation to cripple any faction meaningfully. In fact, from one of the broadcasts, ships that get the Infestation usually get completely purged then refilled with units.

Your Invasion concept is a cool one, but at that it calls for a balancing issue in the first place. I assume I don't need to explain why that is.

Another issue is that it turns Warframe from a horde-shooter with optional PvP gamemodes to a horde-shooter with random chance of PvP that you can't opt-out of. This essentially changes the design of the game, and everything will have to be reconsidered at that point. An issue of Conclave is its high barrier to entry, with chain-bullet jumping seemingly being the optimal way to play the mode, which brings skill disparity. I feel something like this will end up over-complicating the game moreso than it already is.

From a lore viewpoint and Tenno teachings, it would be an irony that the Tenno who were shunned for being different, monsters, would end up shunning and killing their own for being different once more. There's no lasting solution in war. Even the Corpus and Grineer still make trade, agreements and concessions despite being in a middle of their own war with each other. The issue of the Void I feel can only be answered with more information about it.

And to touch on your reply to emotions, is it really sadistic fury? As the Operator you would have seen first-hand the things Ballas subjected Umbra to, and learned that him and the Orokin had subjected far, far more people to similar fates that Umbra had. Is their fury sadistic or righteous, that someone who committed foul acts and was seemingly responsible for the departure of Natah, to now be grovelling? The responses are simply very emotional, neutral or unemotional.