How can a Protestant go to heaven without the sacrament of reconciliaton to forgive serious sin?
Confession is normatively necessary for the forgiveness of mortal sin, not absolutely necessary. We are forgiven for sin as soon as we repent; Catholics know that, as a condition of their forgiveness, they must do their best to go to confession to be reconciled to God for serious sin. As Protestants do not realize that more is expected of them for their reconciliation with God for serious sin than personal repentance and private prayer, they are forgiven upon condition of fulfilling the obligations they understand God to require. The assumption is that if they knew that Christ instituted the sacrament of confession, they would avail themselves of it.
It is easier to see that for the reconciliation of mortal sin confession is normatively necessary (i.e., exceptions to a general rule exist) as opposed to absolutely necessary (i.e., no exceptions) when we consider the case of Catholics who live in areas where no priests are available.
Example: Japanese Catholics lived for centuries without priests after the expulsion of the Jesuits and before the re-opening of the country to the West in the nineteenth century. If sacramental confession were absolutely necessary, we could speculate that many of those Japanese Catholics, deprived through no fault of their own from priests, went to hell for mortal sin they repented but could not confess. Since God does not send people to hell for no fault of their own, we can conclude that sacramental confession is only normatively necessary.
If an exception can exist for Catholics innocently deprived of priests, it can exist for Protestants and others innocently unaware of their need for sacramental confession.