Is it permissible for a person to entrust to God matters that they do not expect to be completed, and will God preserve them, or is that considered ill manners and a sign of discontent with divine decree?

It is prescribed to bid farewell and ask Allah to protect both the traveler and the resident. If the intention is for Allah to protect the husband, safeguard him, and prevent him from traveling to a specific city, even if there is no hope of this occurring, then this does not constitute impoliteness towards Allah nor dissatisfaction with divine decree.