JOEL - In verse 16, "after the manner of" does not necessarily mean "exactly the same as". The way I read these scriptures(v. 16-19) is that the lord was trying to make the work of barge building easy on them by having them follow a basic design of boat they were already familiar with(verse 6). But for this special voyage they were probably given more specific instructions (verses 16-17) that modified the design to accommodate the unusually long voyage in the rough waters of the ocean.
Apparently the modifications were something new to them and God used this opportunity to teach them a lesson on how they can receive His help if they ask for it.

JOHN - And now, in the days of the reign of Omer he was in captivity the half of his days." (Ether 8v4).
If he was still reigning, why didn't he just order himself freed from captivity?

JOEL - Omer was in no position to give any orders because even though he was still technically the ruler, he had been taken captive by his son Jared, who would not have obeyed any of his orders, nor would he have allowed Omer to communicate any orders to his people.

"And when he(Jared) had gained the half of the kingdom he gave battle unto his father(Omer), and he did carry away his father into captivity, and did make him serve in captivity;
And now, in the days of the reign of Omer he was in captivity the half of his days."(Ether 8:3-4)