You asked about if there were reasons why a second surgery might be more difficult. I am certainly not a surgeon, and I have no clue when it comes to Acoustic Neuroma surgeries. But having worked in a hospital for many years, I do know that for some procedures, patients developed scar tissue which sometimes made the second surgery more difficult than the first.
Again, I do not know if this is true with acoustic neuroma surgery, and I would have no idea if the original surgeon was not able to remove all of the acoustic neuroma, if it's possible that scar tissue would have developed around the part that was left behind. And I don't know if radiation could penetrate the scar tissue and be effective?
But you are asking excellent questions, and I think you should ask them to your neurosurgeon and to other specialists you might consult. What I have learned is that if you ask a neurosurgeon about radiation, many of them will play down the effects of radiation. And if you ask someone who specializes in radiation for acoustic neuromas, some of them will play down the effects of surgery. That's why I appreciate the 'team' approach that some hospitals have where acoustic neuroma specialists in both surgery and radiation discuss your situation together. Their answers are more balanced in my opinion.
But again, you are asking the right questions.
Best wishes on your journey!