Does the dryness of a limb during Wudu' mean its complete dryness, or is it sufficient for a small amount of water to remain for it not to be considered dry? And is the dryness of a limb due to being preoccupied during Wudu' considered a break in continuity (muwalat) and thus invalidates it?

"Dryness of limbs in Wudu" means they become parched and free of wetness. Scholars have differed on the ruling of muwalat (succession) between the limbs of Wudu; whether it is a Sunnah or an obligation.

Malikis: If a washed limb dries and the Wudu is interrupted due to forgetfulness, it is permissible to continue and complete the Wudu. However, if the interruption is due to incapacity or intentional action, one may continue as long as the dryness does not become prolonged. Prolonged dryness is estimated by the drying of the limbs of a moderate person in moderate time and place. Hanbalis: Dryness does not matter if it is due to engaging in a Sunnah of Wudu (such as running fingers through the beard or between the toes), or due to fully saturating with water, or removing doubt and waswasah (whispers). Hanafis and Shafi'is: Muwalat is recommended, and Wudu is valid even with interruption. However, according to the Shafi'is, it is disliked if there is no excuse.