Sounds like you need to do a hard reset to get back to square one. I hope that solves the problem. You might need to remove your storage card during the process. The card seems to interfere with ROM flashing.

The 6600 will not run WM5. Several very knowledgeable people here have tried to do it, but as far as I now were unsuccessful.

The USB screen that you describe sounds normal during ROM flash. It takes a while though. So maybe it's a matter of waiting. If you search here, you will find much info about the ROM flashing procedure.