why wouldn't "ela chega do restaurante" be correct?
Becaus it is more like "She is coming from the restaurant" (Ela vem do restaurante.) than "She arrives from the restaurant" (Ela chega do restaurante)
Yes, but it sounds weird. Ela chega do restaurante looks like she arrived from the restaurant.
Why isn't "desde o restaurante" accepted?
desde is for time: desde ontem, desde o mês passado - since yesterday, since last month