Is the recurring obsession a fiancée experiences regarding her fiancé's death a sign of weak faith, a natural pre-marital fear, or a message from God to reconsider?

These whisperings are not an indication of weak faith or a reason to reconsider the engagement. The correct approach is to disregard them and proceed with the engagement without hesitation, as long as you are satisfied with the suitor. One who seeks guidance (through Istikhara) proceeds with the matter after the Istikhara and does not abandon it unless Allah diverts them from it. It is not a condition for the one performing Istikhara to feel an immediate sense of relief after it. We advise you to increase your remembrance of Allah (Dhikr), supplications (Du'a), and the prescribed Adhkar. If the whisperings persist, consult a psychiatrist. The suitor is a stranger to the fiancée until a valid Islamic marriage contract is concluded between them.