In bakarnā bukūran, bukūran is a cognate accusative (mafʿūl muṭlaq) derived from the same root as the verb. It reinforces the early setting out. Bi-saḥrah specifies time; the pronoun in fa-hunna returns to the female travellers.
The comparison fa-hunna wa-wādī al-Rass ka-l-yadi li-l-fam compares the caravan’s closeness to the valley with the closeness of hand to mouth. Its shared property is nearness, not resemblance of shape.