Is it true that the washing of a dead body is equal to the forgiveness of forty-two sins? Where did this concept originate? Does the figure forty-two signify plurality of sins? Since Allah is the ultimate decider and forgiver of sins and all actions are preceded by intentions, which may not be pure, how do we know for sure that a certain act will actually lead to forgiveness?

I am sorry I could not find any such narrative and therefore the reliability of its transmission cannot be analyzed. Please provide me with a full reference. Forgiveness of minor sins granted on any kind of good deed is a manifestation of the abounding mercy of the Almighty. Human beings are prone to make mistakes. The Almighty is always watching over His servants. He knows that a true believer, who strives to lead his life according to divine guidance, may fall into miner sins because of the human element. If that believer seeks forgiveness and is true to heart, then his good deeds expunge the minor sins that he has committed. This we should appreciate is only the manifestation of the mercy and the overwhelming love of God for His true servants.

You have rightly observed that only those acts good or bad will be rewarded which we do consciously and with pure intent. Merely doing small kinds of good deeds, ignoring the greater commands of the Almighty is in fact doing no good at all. These glad tidings are meant only for the true believers who always try their best to lead their life pure and observe all the directives of the Almighty and keep seeking forgiveness from Him for their sins.