Two questions seem to be asked:
1) When can absolution be anticipated to actual confession?, and
2) What happens if the "postponed" confession does not come to pass?
In regards to when our sins are forgiven in anticipation to confession, The Church teaches that this should happen only in cases of physical or moral impossibility (CCC 1484, in part):
"Individual, integral confession and absolution remain the only ordinary way for the faithful to reconcile themselves with God and the Church, unless physical or moral impossibility excuses from this kind of confession."
When absolution of one's sins is absolutely needed --- dangerous circumstances that could lead to death --- and access to a priest for confession is a physical or moral impossibility, The Church advises to make a perfect act of contrition. As noted in other answers to this question (CCC 1451-1452):
Among the penitent's acts contrition occupies first place. Contrition is "sorrow of the soul and detestation for the sin committed, together with the resolution not to sin again."
When it arises from a love by which God is loved above all else, contrition is called "perfect" (contrition of charity). Such contrition remits venial sins; it also obtains forgiveness of mortal sins if it includes the firm resolution to have recourse to sacramental confession as soon as possible.
So, what happens if the anticipated confession does not come to pass?
Well, a close examination of CCC 1452 reveals that, as long as we have made a perfect act of contrition: a) venial sins are forgiven, regardless of whether we confess later or not; b) mortal sins are forgiven provided that, in addition, we make a "firm resolution" to go to confession as soon as possible. Note that only a "firm resolution" to confess as soon as possible is needed, and not necessarily the actualization of such a resolution --- this covers the cases where we die before finding a priest! Of course God knows whether our firm resolution is sincere or not, and he can act accordingly.
As for imperfect contrition: well, in that case we are out of luck if we don't have access to confession. The only ordinary way that The Church knows how to deal with that is through confession --- though God may still in act in extraordinary ways which we do not know --- (CCC 1453):
The contrition called "imperfect" (or "attrition") is also a gift of God, a prompting of the Holy Spirit. It is born of the consideration of sin's ugliness or the fear of eternal damnation and the other penalties threatening the sinner (contrition of fear). Such a stirring of conscience can initiate an interior process which, under the prompting of grace, will be brought to completion by sacramental absolution. By itself however, imperfect contrition cannot obtain the forgiveness of grave sins, but it disposes one to obtain forgiveness in the sacrament of Penance.