Is the sin of going to a soothsayer waived if the motive behind it was to seek an interpretation of strange past statements, not believing in divination or asking about a prophecy, and does this entail the non-acceptance of prayers for forty nights?

Severe warnings have been issued against approaching those who claim knowledge of the unseen and believing them. An ignorant person might be excused for the unlawfulness of this act, and the sin lifted from them, as we have no way of knowing what Allah has decreed. However, you should repent and seek forgiveness for what you have done, for a Muslim is commanded to repent in all circumstances. It is never permissible to abandon prayer, and the meaning of prayers not being accepted for forty nights is that one will not receive the reward of Allah's pleasure and honor, not that the performance of the prayer is waived or that its make-up becomes obligatory.