If I can run sub 17 off 30-35 mpw, how fast can I run with 70+?

a lot faster.

Quantity doesn't always lead to quality. There's a fallacy in that. I'd slowly increase your mileage overtime but keep the general efforts near the same as you're doing in your 30-35 miles per-week. You can't simply use the logic that because you're doing well at 30-35 you'll do just as well at 70+. It all is going to come down to how your body absorbs the mileage and recovers. There's a lot of kids who commit to the higher mileage principal but yet could probably put in larger amounts of consistent efforts at 25-30% less work.

Here's what I say to every high schooler asking for advice on here: Talk you your coach about your training. Do not allow Letsrun to train you.

That said, from the information you provided you will be faster barring any injuries. If you held your mileage constant you would continue to improve as you mature.

Also don't be set on 70. The kids I knew in high school who did 70+ were way better than me at the time, but once they got to college they slowly burnt out or got injured while I continued to make large improvements even in my senior year. By that time I strategically built up to 100 after doing maybe 50 in high school.

I tend to respond very well to LT intervals or a constant 5-6 miles at LT pace. This is what helped me make big jumps during track, and also increasing my long run. I don't know about overall mileage, but I currently do a 10-12 mile long run and a 4-6 mile LT run each week with some hills or a fartlek. I'm doing about 55-60 mpw right now.

You will run the same or worse. Depends when the dead legs, stress fracture or reaction occurs.

I'd hold it constant at 55-60 if your body is adapting well to that. Just remember to have drop recovery weeks to avoid burn out and injury. Maybe 2-3 summer 5ks to test mileage benefit but I wouldn't encourage you to 'race' during the off season only to use the races as more of a litmus test to access your speed.