Hi lat - congratulations to you and your husband for getting through the very tough treatment that so often comes along with any head and neck cancer diagnosis. As I've been told, and can attest to personally, being
HPV+ is an advantage with this disease (versus the older and more traditional smoking-caused cancer). When I finished treatment nearly 7 years ago and asked my RO whether I was now in remission, she responded, "Oh no, you're cured."
So it is indeed possible. And although there's always a tiny voice in the back of one's mind reminding one of possibilties (nothing's ever 100% in life as we all know), I've basically stopped worrying about a recurrence.
I recommend reading some of the survivors' stories on the appropriate forum.
My thoughts are with you all, and especially with your mother. I know this must be very difficult for you and for her. Please keep us posted, and ask any questions that come up. We're here to help.