It sounds like Harley's veterinarian is pretty sure it's a histiocytoma. Then again, a biopsy would certainly confirm it one way or the other...
A few months ago, Ozzie had a histiocytoma on his muzzle that was about the size of a pencil eraser. At the time, the local veterinarian did do a fine needle biopsy that was negative, and he prescribed a synotic solution that I applied to the lesion 2X a day.
I'm still not sure if that solution really did anything, but after about 6-8 weeks the bump did go away. Maybe it was just time that made it eventually go away...I'm not really sure.
Regarding surgical removal, I think the approach with histiocytomas--assuming that's what this bump really is-- is to wait and hope they just go away. I certainly wouldn't want to have any surgery done for a bump that still may go away over time, but again, you may want to consider eliminating the guesswork and having the biopsy done.
I wish you guys the best. Hopefully, time will run it's course and the bump will just go away.
Bob.