You are attempting to judge that which cannot be known except between the individual and God, barring supernatural gifting. Perhaps motives is not totally an accurate term, but you are trying to judge upon them something that is beyond our ability and with our current knowledge.
We can only judge results. A truly sincere "prophet" (if there is such a thing in this day and age) would quickly understand their error and move to resolve it with the church. The church should be trained to discount their false messages and relegate them to obscurity.
You have no arguement from me there. And upon the basis of results we are commanded to judge. The unknowns are God's realm, and whether it was a sincere misunderstanding of what God told them or satan's whispers, is something we do not have the ability to judge and ought to leave it between God and these prophets.
I don't have to judge motives. The OT prophets were never judged by motives. If they failed in their word it was a sure sign that they were deceived. Period. End of discussion. It was a sure enough sign that they could be killed based on that fact alone.
I know the options and I have laid them out here. Every one of the options is fatal to the "prophet". That's why God instituted a strict standard by which His church could judge a faithful word from the Lord: 100% accuracy every time they open their mouth. And that's also why the punishment for inaccuracy was so severe. It's easy to dupe the people of God, especially when the people are unwilling to deal appropriately with the offender. We are much to sentimental.
We obsess with man's standard rather then the Lord's. We have a sure sign that these are false prophets, yet some continue to try to rehabilitate them and their message. Why is that?