A non-determinist view of time does not only mean that the past is gone, is nothingness; it also means there is a second order of nothingness - the past that could have been but was not. What is the nature of this difference in non-existence: the difference between what was but is not, and what might have been but never was? It is hard to imagine a distinction between two nothingnesses, but the meaning of our existence - our redemption in Eliot's terms - rests on this layered nothingness.
What might have been is an abstraction
Remaining a perpetual possibility
Only in a world of speculation.
What might have been and what has been
Point to one end, which is always present.
The second sentence has an overt meaning: both the past that was and the past that might have been are functionally compatible with the present: they both 'point to' the present. It also introduces the idea that the current moment is eternally ending, always passing away, which is expressed throughout the Quartets. You could just as well say that the current moment is always refreshing and renewing itself, always coming into life, but Eliot is sucked towards the darker bleaker half of this duality. He is always pulled towards ending.
Footfalls echo in the memory
Down the passage which we did not take
Towards the door we never opened
Into the rose-garden.
Our freedom rests on the doubled non-existence of the choices we did not make: the past that not only is not but never was. Typically, he expresses the reality of free will and the openness of time in terms of good things that were not done. The reality of time could just as well be expressed in terms of good choices - I mean logically it could. By the temperament of the writer, and the context of the poem, it could only be this way.
The rose garden, here and in the next section, represents innocence and enlightenment. Our free will allows us to choose not to go into the rose garden - or in Christian terms, it has excluded us from Eden.
What strikes me most about lines 1-14 is how compact they are, and how efficiently they convey meaning. They are not beautiful lines, they are stripped to a bare scaffolding.