There's no duty to obey an unlawful order; UCMJ case history indicates that in order to be lawful an order can't infringe on the statutory rights of the person receiving the order. In other words, you don't have to kill yourself attempting a patently impossible feat, and as there can be no possible military objective being pursued by giving the order, the officer should know better than to issue it.

As for your second scenario, yes, that's about it. Although to be honest it's unlikely to ever go to court martial; an officer demanding impossible things happens all the time and is dealt with at the company level.