In the above, written two years ago - I said that having not understood the concept of retrospective prayer, in - for example - Charles Williams' novel Descent into Hell - I now (as of two years ago) finally understood it.
As of now I believe that the concept of retrospective prayer, and indeed the Boethian framework which rationalizes it, and describes God as out-of-Time, does not make sense - or rather, that it:
1. contains an incomprehensible necessity for transitions between God's world out-of-Time/ in eternity; and mortal human life in Time,
2. entails an ultimate monism and stasis in which change and free will is an illusion, and (among other things) human mortal life is rendered a futility.
I now believe that - as common sense implies - Time is linear, events are irreversible; and once something has happened, it cannot be undone (although it may be healed, and indeed it is this promise of healing which is near the heart of the Christian Gospel).
A belief in retrospective prayer will not stay-put - but its implications ramify and erode the vital importance of now - erodes the reality of free will - disperses the necessity of bringing of matters to the point of choice.
Having tried to live with the implications that retrospective prayer is valid I find that it is a confusing, paralysing, demotivating theological idea.
I think the idea of retrospectively-effective prayer is one of those brought in to deal with unacceptable consequences of other false theological principles - so I believe it can be discarded without bad consequences.
And I have therefore discarded it. Let's see how this works out...