Internal consistency in prophecies about Ammonihah

Alma and Amulek preach in the city of Ammonihah, where there is much wickedness. In Alma 9:18, Alma says:

18 But behold, I say unto you that if ye persist in your wickedness that your days shall not be prolonged in the land, for the Lamanites shall be sent upon you; and if ye repent not they shall come in a time when you know not, and ye shall be visited with utter destruction; and it shall be according to the fierce anger of the Lord.

In Alma 10:22-23, Amulek says:

22 Yea, and I say unto you that if it were not for the prayers of the righteous, who are now in the land, that ye would even now be visited with utter destruction; yet it would not be by flood, as were the people in the days of Noah, but it would be by famine, and by pestilence, and the sword.

23 But it is by the prayers of the righteous that ye are spared; now therefore, if ye will cast out the righteous from among you then will not the Lord stay his hand; but in his fierce anger he will come out against you; then ye shall be smitten by famine, and by pestilence, and by the sword; and the time is soon at hand except ye repent.

The people have been warned: If they don't repent, the Lamanites will visit them with utter destruction. It has not happened yet, only because of the prayers of the righteous. But if they cast out the righteous, the Lord will not "stay his hand".

What did the people of Ammonihah do? Alma 14:7:

And they spit upon him, and cast him out from among them, and also all those who believed in the words which had been spoken by Alma and Amulek; and they cast them out, and sent men to cast stones at them.

Alma 15:15

But as to the people that were in the land of Ammonihah, they yet remained a hard-hearted and a stiffnecked people; and they repented not of their sins,

Basically exactly what they were warned against.

What happened next? Alma 16:2-3

2 For behold, the armies of the Lamanites had come in upon the wilderness side, into the borders of the land, even into the city of Ammonihah, and began to slay the people and destroy the city.

3 And now it came to pass, before the Nephites could raise a sufficient army to drive them out of the land, they had destroyed the people who were in the city of Ammonihah

Basically, exactly what was prophesied