Dear Tower,
Your case seems to be fraught with uncertainty and complexity. I would suggest getting a third opinion to see if there can be some concensus. Given the time constraints that you mention, it might make sense to go to MD Anderson in Houston (or another comprehensive cancer center in the US) for quick turnaround on an additional opinion.
Generally, I would suggest the most aggressive treatment possible, as squamous cell carcinoma is an aggressive cancer (assuming your diagnosis is correct).
The need for IMRT radiation is something that is puzzling in this case. Since it is used to target a tumor site very specifically and to spare surrounding tissue, I don't understand why this would be the preferred method for you, if indeed there is an unknown primary. I'd think that a radiation technology which bombards the entire head/neck area more widely would be more appropriate given the lack of a precise target. If surgery isn't an option because of the fear of spread, I would think that IMRT radiation would raise the same concerns.
I'm not a health professional so this is conjecture on my part. I wish you all the best in pursuing your treatment options. - Sheldon