In simplest terms, I don't think the HUMANITY (AND VASUDANS) **** YEAH is a realistic option even in INF. I remember the old talk about how to kill the Gargant, and even then it wasn't really possible. I also don't see it ending with the psuedovictory either though, that'd be out of character.
There can be lesser degrees of victory than total, without being pyrrhic. The Shivans can simply pull a disappearing act again, as they have before. They can present the Alliance with something that cannot be defeated without gutting the fleet, and vice versa. A silent equality of arms, staring at each other across a jump node and knowing that neither one can afford to cross the line, would be a unique way to end.