Can we apply Dalton’s law of partial pressure to a mixture of carbon monoxide and oxygen?

No, the law cannot be applied because these gases readily combine to form carbon dioxide. The law can be applied only to the non reacting gases.

Why a bottle of liquid ammonia is cooled before opening the seal?

If the seal is opened without cooling, the liquid ammonia suddenly vaporises and gushes out of the bottle with force. This may lead to serious accidents.