This is not something I have ever had experience with, so I'll leave the advice to the vets and vet techs on here or others who have experienced it. Having said that, you are correct in your assumption that some of what the "S/N fanatics" say is bunk. I have always spayed my females at what ever point they either need it or I am no longer going to breed them. So, in other words, many were "adults" and had gone through heat cycles. Grip, for example, was 6 years old before being spayed. She died at 14 from lung (or throat, can't remember now) cancer. My point is, I have never had a bitch die of or even get mammary cancer. I have, however, had a dog I neutered die from prostate related issues. So, you are wise to question the advice of many non breeders about S/N. No one understands their concerns more than I!!!! Lord, I had to kill dozens of dogs a week for a living, so I am a HUGE pro S/N person. But I also believe in being honest about the risks and benefits pro and con.
Best of luck with your girl. Is this something you can take a picture of and share? It seems quite rare, as I'm not even sure I have heard of it, so it would be awesome if you could post it for educational purposes. Thanks!
Ch Bandog Dread SchH 3 IPO 3 TD CD UCDX WDS STDd/s WH
Boldog Dirk SchH 3, Brevet
Bandog Brittania SchH I WDS UCD
Boldog Grip SchH I WDS Silver Medalist
Boldog Grim SchH 2 Regional Ch
Boldog Old Yeller SchH I IPO I, STP 1
Boldog Hellboy FH CD TD