Hi 48 Below,
If 10% was left, it would be a pretty small bit, maybe 5 mm in size. If it is growing again now, it is probably still only 10 mm or less in size. It is also most likely that the bit left behind was in or near the canal, or IAC, meaning it may not be near the brainstem at all. That makes radiation treatment a real possibility. I hope your doctor discusses that option with you, or that you seek out an experienced radiation oncologist for an opinion on that kind of treatment. Given your first experience with surgery, it seems like considering radiation this time would make good sense.
I don't see why you can go take a hike up a mountain, there should be no urgency this time. It has taken 4 years to come back, and is almost certainly much smaller this time. Waiting 6 weeks should make no difference, and it would get you in good shape and good spirits as well.
Best wishes, and let us know what you find out,