Vera has had a couple of rechecks since I last posted. The veterinarian we're seeing still thinks it's a Zymbal's Gland tumor. She said she's seen many ZG tumors and seems very familiar with them. My veterinarian used to work in a practice where the owning vet would remove ZG tumors and she also did a few herself. She does not recommend doing this, however, and says the tumors usually grow back and the surgery is very invasive and the recovery quite difficult.
Vera's vet said the tumor will probably eventually grow out through her cheek. My impression is we were able to diagnose this tumor earlier than most since it hasn't already spread to the outside of her face. It was diagnosed because Vera fell off of a bathroom counter which she previously had maneuvered with ease and would never have fallen off until this tumor developed.
So far, my sweet Vera has been kept comfortable and the tumor growth has been minimized thanks to her medications. She's taking Amoxicillin, Baytril and theophylline (for respiratory symptoms which are no longer present) as well as prednisolone. Vera's gotten more wobbly and weak but seems in good spirits and is eating well.