Will a man be rewarded if he marries a divorced woman older than him for the purpose of protecting her chastity, and will his wife be rewarded for her patience and acceptance of this marriage?

Your marriage to a righteous woman with the intention of protecting her chastity is a good deed and an act of obedience for which you will be rewarded, and similarly, your wife's patience with a co-wife [co-spouse] will be rewarded. A Muslim is rewarded for doing good, as Allah Almighty says: "So whoever does an atom's weight of good will see it" and "And whatever good you do - Allah knows it." Justice between wives is a condition for polygyny, so whoever fears not being able to act justly, it is not permissible for him to proceed with it, as Allah Almighty says: "But if you fear that you will not be just, then [marry only] one."