Does a secret marriage proposal from a widow during her waiting period (iddah) prevent one from marrying her after the iddah ends, based on the principle of "whoever hastens a matter before its appointed time shall be punished by being deprived of it"?

It is not permissible to openly propose to a woman observing her iddah (waiting period), whether she is observing it due to divorce or death. Both of you erred in what transpired between you regarding the open proposal and acceptance. However, there is no sin upon you if you were ignorant of the ruling. Your marriage to her after the expiration of her iddah is valid. It is more appropriate to say "Allah willed" (شاء الله) or "Allah decreed" (قدّر الله) instead of "circumstances and fate willed" (شاءت الظروف والأقدار).