Dancer, I forgot to reply to something.
+1 Xylitol! Proven to fight decay. It intervenes with bacterial colonization and acid production. Good on you for knowing about Xylitol. Anyone with opiate induced xerostomia should also be chewing gum (xylitol gum) throughout the day. Promotes saliva production.
If brushing is not an option, definitely rinse mouth with water after eating/drinking anything with ANY form of sugar or acid. Things like meat, nuts, and cheese actually dont allow bacteria an edge, so those are teeth-friendly foods. But hell, even use your finger while swishing to wipe off teeth. It's all about interfering with bacterial adhesion to the teeth.
Limit intake of sugar laden foods or soda to the time right before you brush. Eat/drink these things as quickly as you can. Do NOT sip on a soda all day. THis just keeps your enamel demineralized and softened for as long as you sip. Chug it, and ALWAYS use a straw to keep it away from your teeth. Brush or at least rinse with water immediately after. Or swish with flouride rinse.
Every single sip of soda demineralizes enamel and acidifies saliva for 20 minutes before pH of saliva can buffer it back to neutral. Each sip of that soda, even diet (remember: acid), makes your teeth an orgy-fest for decay causing bacteria. Sip on that soda or multiple sodas for X amount of time... just think, 20 minutes after each sip, your teeth are screaming in acid laden agony. So, again, sip responsibly - with a straw. or better yet, chug it.
Flossing is SO important. If you were ever trapped on an island and could only have either a brush or floss, you'd want the floss. This is because of the nasty bacteria that hide out in between teeth. This is not only the easiest place for cavities (in bw back teeth) but flossing disrupts the bacteria that sit under the gumline, the ones that cause gingivitis, periodontal disease, and bone loss of the jaw.
Brush at least 2x/day, better yet after EVERY meal. toothpaste optional for mid-day brushings. Floss at least 1x/day, always at nighttime. Second priority would be first thing in the morning as you wake.
Don't wait til after breakfast to brush/floss if there's a chance that you WONT after breakfast. Since saliva production nearly halts in our sleep, that means that bacteria colonize like crazy while we sleep. Plaque is millions of bacteria stacked on top of each other, not food debris. Use your fingernail against a front tooth when you wake in the morning. That white/yellow plaque shit is literally 1 million bacteria stuck to your fingernail. That's why you need to brush in the AM -- people will say well.. "i brushed before bed and didnt eat or drink anything while I was asleep!", yeah - logically sounds legit, but bacteria will colonize regardless. even without eating sugar, we still leech sugar into our saliva from our blood. this is why we have a constant blood sugar reading. this is why uncontrolled diabetics can have gum disease. their shitty blood sugar levels leech our a crazy amount of sugar into the gums. to top it off, diabetics immune system blows.