If adult bees don't efficiently digest pollen; how do they get their essential amino acids?

Two ways: from the production and consumption of bee bread and from the jelly fed to them by young bees at the jelly producing stage of their progression in duties, polyethism. This is the stage at which those young adults mix the secretions of their hypopharyngeal and mandibular glands, varying the secretion mix according to whether they are feeding larvae, queen, drones or workers.

Bee bread is the other way that older larvae and adults get their protein. The bees ferment pollen in cells of the comb by mixing in enzymes and microbes in the regurgitated nectar brought in by returning foragers.