Galaxy at War's "Galactic Readiness" meter is... a bit controversial. You increase it by playing ME3 Multiplayer, and by playing an iOS game that acts as a tie in. The 'Readiness' score that accumulates from these activities has a floor or 50 percent, which functions as a .5 multiplier on your troop strength as you prepare for your final assault against the Reapers. At 100%, it's a 1.0 multiplier. Straightforward enough.
Interestingly, if you look at all of the available thresholds for the games various endings, they span a range of readiness scores that is slightly wider than a band where the top end is double the bottom. If this had been the case, it would be possible to get to the endgame with exactly the needed resources for the best ending, and then beat the game at various readiness levels between 50% and 100% in order to see all of the endings.
Instead, because of where the mathematical breakpoints are, if I wanted to see every ending on one playthrough, I would need to finish with 3499 readiness (this is the maximum you can finish with at 50% GaW rating to see the 'worst' ending, which occurs at below 1750 after the multiplier), however, this still leaves two break points, at 4k and 5k unreached.
Unless, it is somehow possible to get the GaW meter to exceed one hundred percent and bring it to 143% or more.
If this is possible, suddenly it becomes an option to finish the game at exactly 3499 and see every ending in one playthrough by manipulation of GAW. If this is not possible, then seeing every ending (along a given branch, there is one major fork that is based not on readiness but on an imported decision) is not possible with a single playthrough of the game without replaying the entirety of the final mission before and after a bunch of tedious planet scanning.