The current is so far you've been: Shot, blown up, shot again, been thrown around before finally slamming into filth, shot again, mindraped, blown up again, derezzed by a laser, and then being crushed under a bunch of rubble. Your first thought should be, "Wow, I really am Space Jesus," but you can chose, "I know I've made it through worst before, but logically I could die from falling rubble, even if I'm illogically alive now."
The current situation is the “now,” not the then”:
“Currently” Shepard is severely wounded, bleeding, being in a nearly destroyed station, etc.
“Then” is what you are using to validate your assumption: “she survived all that before, she must survive this too now, regardless of the current dangers.”
You are using what happens in other scenes as a rule to validate what happens “now”, (and invalidate any other concern). Unfortunately, if we use other scenes to validate what happens in that particular scene we also have to consider that Shepard’s death is possible, because it happened before once, could have happened twice (suicide mission), and could be happening now anyway, if you picked any other colour but red. The obvious conclusion:
Survival in some circumstances doesn’t guaranty survival in all circumstances. “If Shepard could die before, there is no guaranty she can’t die now, regardless of how unbelievable some of the things she went through.”
Your choice reflects a personal perspective, and it is a valid approach, but does not address the problems as viewed from different perspectives, and it certainly does not invalidates them.
Not really, Reapers were flying around the Citadel well before the beam went off, and cruisers were scrambling to speed out of there. There's a higher chance of them hitting Shepard than the Citadel somehow facing any worse damage. If life support were failing anytime soon, the actions that make breathing possible would not be happening. Malfunctions don't happen suddenly and abruptly with this level of damage, They would last long enough for someone to locate Shepard, or they would have shut off earlier. Considering that gravity still seems to be a thing, I doubt the systems are failing. And sure, Shepard could bleed out in a few hours, but first you would have to prove that they would not be found in a few hours.
That is an awful lot of assumptions. Now, there is nothing wrong in making them, considering the ending is pretty ambiguous and open to one’s personal preferences, but that does makes it a personal take… me? I still consider exponentially more likely to die of heavy trauma and massive bleeding than a reaper falling upon her. Mind you, the onus of proof that Shepard must be found in time is of anyone that insists in it being so. I and others have already enumerated a good number of factors that clearly hinder that.