But the one and only true fact is... ZERO IS DEEEEEEEEEAD!
Just like Batman's parents.
I need a Ceil (in tears) shouting "My boyfriend is deeeeead!" slapping a newly-modified Alouette. Seriously, someone has to keep the peace before new heroes can be born. She'd prolly became "DarkSwan" or some [parasitic bomb] like that to fill in the 200 year gap.
X6-X8 + CM are cannon because if they weren´t The Zero series would look completely different!
The guardians were supposed to be alive after Z3...but then came ZX and they had to be dead. Because they were in a situation they could whether survive or not, INti decided to let them die (assholes).
Their deaths I'd have to say.. Good riddance. I never liked Leviathan.

And Serpent is a Repliroid; pay attention to the red triangle.
Yep, the triangle really can tell them apart, can't it? Even if it's in it's proper position (itt: it's upside-down).