Sorry for your troubles. It's not good to have ECE, and so many nodes involved, which increases the chance of recurrence, distant metastases. The good part is the lymphs nodes were removed, but there are so many lymphs in the H&N, like 300, they probably did not get all being most are microscopic, so Chemoradiation is needed to fully eradicate any cancer which may still be there microscopically, and HPV is very responsive to this treatment, and or another type of treatment too. You mention surrounding tissue invaded, was this found or saying is a possibility.

I had 70Gy to the primary, 62Gy to the neck, and still had recurrences. They have to use more than 50Gy to kill SCC. I see yours is 60Gy, I guess to the primary, and would assume a lower dose is for the cervical neck, maybe 54Gy, which seems some are doing in clinical trials, and wonder wonder if this is enough with ECE, which dosage is usually increased, and outside clinical trials most dosage is 66-70Gy. Maybe you can ask about this and or a 2nd opinion.

Good luck.