Hi! I'm checking in with the ANA discussion board for the first time in a quite a while, and I saw this thread. I am so sorry for what you're both going through -- I know how rough it is. I too had retrosigmoid surgery (at age 39), and I too have persistent headaches -- but, thankfully, they're now well controlled with medication. Sharing what's helped me in the hope that it might give you or your doctors some ideas:
It's a long story, so I'll spare you the details, but through a mix-up about my current medications (aka total stroke of luck), I was put on gabapentin when I wound up in the hospital a couple of weeks after my surgery. I remained on gabapentin for a few months, until I was weaned off it for some seizure testing. The big discovery was that without gabapentin, I had non-stop crushing (non-migraine) headaches. On gabapentin, I still experienced headaches, but they were far less severe. Once I returned to work and had to use my brain, gabapentin 3x/day no longer cut it, so my neurologist increased it to 4x/day. That helped but I wasn't great at keeping a steady pill schedule because 4x/day broke the breakfast/lunch/dinner routine. My neurologist recommended seeking out a true headache specialist. I told her about the pain I experienced in the troughs between doses, and she suggested Gralise, which is a once-daily, extended-release form of gabapentin. (She had actually never prescribed it before because it's so much more expensive than regular gabapentin, but luckily my insurance plus a manufacturer's coupon gets it down to about $25/month.)
And Gralise is, for me, the golden ticket. I take 900mg once a day around dinner time, and it allows me to get out of bed and function in the morning instead of having to shuffle around in pain until the first dose of gabapentin kicks in. Unfortunately, I experience a multihour trough before each day's dose, so my doctor ended up prescribing one 300mg dose of regular gabapentin as well, to be taken in the afternoon. And that combination is great for me. (It confuses pharmacists, though -- they always ask if I'm really and truly taking two different forms of gabapentin.) I still have breakthrough headaches, but not nearly as often. This is not to say that this funny combo would be right for anyone else -- it just happens to do the trick for me. Gabapentin/Gralise definitely make me tired, but for me it's easier to live with that than the headaches. I don't know if your headaches are migraines -- mine aren't, and I know migraines are a whole different situation.
2) Neck physical therapy
When I first saw the headache specialist, she immediately noticed that I was holding my head in a funny way -- somewhat off my spine, both tilted and twisted, so my muscles had to do the work of holding my head up. She said this is fairly common with retrosigmoid surgery, due to the awkward position the head is in during surgery -- apparently it can really mess your neck muscles up. I was aware after surgery that my head was always cocked to the side (like a curious dog), and I thought I'd gotten it back to normal, but it turns out I was a terrible judge of normal posture. She prescribed neck physical therapy, and after a couple of months of PT my head was back in its proper place. They also helped me understand how to hold my head while reading, which had become a very painful activity.
Like gbkim86, I regularly got massages pre-COVID, focusing on tightness in my back, shoulders, neck, and scalp. I definitely see a period of relief from that, until the tension starts creeping back in. At first I got them every 1-2 weeks, but eventually I was able to switch to every 3-4 weeks. Due to COVID I haven't had any in months. I was terrified of what I'd face as a result, but with frequent neck exercise it hasn't been so bad. (I also attribute some of that to no longer having to take NYC subways every day during rush hour, when you're almost always forced into some kind of awkward posture.)
All that said, please don't let my 4+ years of headaches scare you! I had several complicating factors -- bacterial meningitis, which necessitated a second surgery to remove the titanium and wash out my brain, which then required removing the infected skin around the incision and pulling my scalp even more tightly in order to re-stitch me up. I've heard that not having a barrier between the brain's dura and the muscle over the skull can lead to headache-causing adhesions, and now I have an extra-tight scalp too. So who knows.
Don't give up on trying to get in with a good neurologist, especially one with a headache specialty! Even if you have to take an appointment many months in the future, you may also be able to be put on the waiting list in case of cancellations.
Good luck, both of you!