Doctors are different - thus, different opinions on treatment options (as well as surgical approaches, if they do surgery).
Most doctors will recommend what they do best, which is why it's good to talk to a doctor who does radiation as well as surgery, or at least talk to one who does surgery and one who does radiation. You definitely want a well-rounded perspective of everything.
I don't know how deep into the IAC my AN was but it was on the 7th and 8th cranial nerves, if that helps. I had the retrosigmoid approach and my AN was removed "entirely". Entirely being a somewhat subjective word. Although docs do their best and remove what they can see of the tumor, even one cell left behind is enough for regrowth to begin. That said, the chances of regrowth after surgery where the tumor is entirely removed are only 1 - 2 %.
As Laz said, no doctor can guarantee you anything; like you and me, they are only human. IMO, if you find a doctor who guarantees you anything 100%, you should look elsewhere because he/she isn't being completely honest with you.
I'm not sure if mid-fossa vs retrosigmoid would give you a better chance of tumor removal or not, that's probably a matter of opinion based on the doctor you're talking to at the time, but mid-fossa is the best approach if you have good hearing and want to save it.
I'm not a doctor, but I don't think you're too young to radiate. So if you just don't want that treatment option, I'd base your decision more on your personal preference than on your age.
Best,
Jan