#1 may be true for some babies (it's more often true for MOM!), but seeing as how most babies in the world co-sleep, it's usually not true at all and does not seem to be true for your dd.
#2 is not true unless your baby has a rare cavity-causing virus. (Check the Mothering article archives for a great article on this topic. Probably from about 2002 or 2003.)
#3 is probably true if you hope to get her out of your bed within the next couple of years. If you don't mind co-sleeping for a number of years, then don't worry about it. If you *do* mind, then know that you *can* transition her out earlier, but it will be harder. But since she's been in bed with you for 6 months, it's going to be hard anyway, no matter when you do it, so I see no reason to do it before you are ready. Besides, at least a toddler can understand *some* of your explanation about sleeping in her own bed. A 6 month old only knows you aren't there.
It may be too late at this point with this doctor, but my main piece of advice in dealing with these kinds of questions from you child's doctor is to not offer any non-medical information you don't feel you need advice on. Example:
Ped: "How is DD sleeping?"
You: "Fine. I'd say she sleeps about 10 - 12 hours a night, plus __ hours in naps." STOP. IF you want to be chatty, you might add "She has her nights, like all babies, but nothing we can't handle."
The medical concern is whether she is getting adequate sleep and that the sleep environment is safe. (I assume you've assured her that you have no gaps the baby could fall into or fluffy pillows, etc.) The rest is the ped's personal opinion about best parenting practices. Unfortunately, you have now gotten into the conversation and your ped will probably not stop the questioning there at the next appointment. She will likely follow up with, "Is DD still in your bed?" You'll have to think about how you want to answer this. You have 3 choices:
1) You can lie outright: "No, she sleeps in her own crib now."
2) You can give a half-truth: "She has her own crib and she likes it." (Not revealing that what she likes is to crawl under it play peek-a-boo, but she never sleeps in it.)
3) You can tell the truth and let your ped know that you have heard her advice and are rejecting it. "No, we still have her in our bed. We discussed your opinion and looked at some other advice as well and decided to let her stay there for now. I feel confident that she is getting enough sleep."
If you feel strong enough, I highly recommend #3. It will help to broaden your doctor's mind, or at least make other co-sleeping parents seem less odd to her. Plus, it's good practice in speaking up for your own authority as a parent. Where your child sleeps is *your* decision, not her doctors.