What argument moves Achilles to go back in the battle? What happened between Diomedes and Glaucon? Do you see any connection between these two actions?

It is the death of Patroclus that drives Achilles back into battle.

Diomedes and Glaucon fight, but they exchange armor before their battle. They do this because Diomedes’ grandfather was a guest/friend of Glaucon’s grandfather.