Examining the account of Jonah going to Ninevah tells us that the prophecy was not false. The purpose of any discipline given by Jehovah is to cause a change of heart. 1Timothy 2:3,4 which states, "This is fine and acceptable in the sight of our Savior, God, whose will is that all sorts of men should be saved and come to an accurate knowledge of truth." It is obvious from the conduct of that city prior to this time reveals that she (ninevah) did not think they were so bad. So the message delivered by Johan with Jehovah's direction had to be stiff to get their attention. Just like some instances with children require a different tactic to get different one's attention.
Ninevah definately had a change of heart. It would have been out of character of the true God Jehovah not to react accordingly to their true repentance.
Wouldn't we hope he would do the same for us. Perhaps after he reads these statements he may feel you do not appreciate his spirit anointed FDS and like Miriam question his wise direction in useing imperfect men like Moses and today the FDS to give direction to his people. Grieving Jehovah's Holy Spirit as did Miriam could lead to disasterous consequences.
It is only natural that we all want an end to this system of things. Satan plays on that natural desire to work on our own impatience and imperfection to erode our trust and loyalty to Jehovah. After all that is what Satan did.
Please, be patient as in every case previous to this, clarification will come. But in Jehovah's time and direction not our own.