How can one reconcile the consensus on the absolute prohibition of loans with an incremental condition, with the action of the Messenger of Allah, may Allah bless him and grant him peace, in borrowing charity camels with an increase over what he took?

There is no contradiction between the consensus on the prohibition of every loan that draws a benefit and the actions of the Salaf (early Muslims) in selling a camel for two or more camels on credit. This falls under the category of selling and Salam (forward selling). It is reported that Ali ibn Abi Talib, may Allah be pleased with him, sold a camel for twenty camels on credit. It is permissible to sell items of the same type for unequal amounts on credit if their purposes differ. It is also permissible to conduct a Salam transaction for an item of its own kind, provided there is a multiplicity from one side, or a difference in benefit, or in size (small or large).