First, Jeremiah was not an open air preacher. He was a prophet and he was called to bear witness against Jerusalem. The point is not Jeremiah's faithfulness (although he was faithful) but Judah's unfaithfulness and rebellion. You are taking Jeremiah as out of context as you would should you decide your relationship with your wife serves as a witness along the lines of Hosea. And this is one of the biggest problems I have with Ray Comfort (and WOTM as you've presented it here). We cannot ignore context for support of our actions.I am busy reading Ashamed of the Gospel 1993 edition.
Jeremiah preached (open air) 40 years without positive responses and had many other problems as a result of his preaching.
What I have experienced in Denver is much rejection. But I have not yet been persecuted for my evangelism.
Second, NOWHERE in Scripture do we find faithful Christians except that they are discipled and being discipling others. The Bible assumes that those who are saved are added to the church. I'd urge caution in trying to "lone wolf" evangelism. It is not biblical. Your personal ministry has to be an extension of your church. If they do not want to do it your way, then perhaps you should try their way (and see what you can bring to the table).