can you pass the ball forward behind the srimage line,then pass the ball foward across the line of srimage

YES

You can even throw it over the line, then back, then over again/never seen it in writing. There are a lot of urban legends out there!

It is not an urban legend. I have seen it done before. Best move is to put the ref on notice before the game that it may be coming. You run into less issues that way. As you know, sometimes the refs are not up to speed on the various rule nuances.

In the IFFL, you CAN NOT throw it over the line, then back, then over again.