A quick update. When I asked my mom's pcp how long she'd been taking Fosamax, she said my mom was due for a "holiday". It had been just over 5 years. Not as long as I'd thought, and she was not as cautious as I'd expected... So my mom will start with a 6 month break, then possibly another 6 depending on her next DEXA. She said an endo wasn't necessary because my mom's done well with Fosamax. This seems a good enough outcome to me, and doesn't complicate my mom's life.
Thanks so much for the good advice and support!