In wa-idhā taʿālā laḥmuhā wa-taḥassarat wa-taqaṭṭaʿat ... khidāmuhā, idhā introduces a recurring or conceivable condition. Coordination links three processes into a single sequence.
Laḥmuhā is the subject of the first verb; khidāmuhā belongs to the last. The feminine subject of taḥassarat refers back to the she-camel. Grammar thus unifies mount, body, and equipment in one scene.