A good write-up!
Donnie gets out to ask her if she is all right, and she whispers something to the effect of, "Every living creature on this planet dies alone,"
Actually I think she says "every living thing has one".... talking about the water-looking things coming out of people's chests.
SPOILER BELOW!!!!
I get the impression that the Tangent universe wasn't created to prove anything to Donnie, but was created via a rip in space and time. He was chosen by god and guided via Frank and those watery looking guiding things to bring it back into existence again before it collapsed and destroyed the universe, ie.,
The Tangent Universe
--------------------
The Primary Universe is fraught with great peril. War, plague, famine and natural disaster are common. Death comes to us all.
The Fourth Dimension of Time is a stable construct, though it is not impenetrable.
Incidents when the fabric of the fourth dimension becomes corrupted are incredibly rare.
If a Tangent Universe occurs, it will be highly unstable, sustaining itself for no longer than several weeks.
Eventually it will collapse upon itself, forming a black hole within the Primary Universe capable of destroying all existence.
So maybe the act of the engine falling onto Donnie from the "real" world into the tangent world seals the rip in space / time and saves everyone?
My head hurts.