As was noted above, experience is critical. The way things are going in the health care industry, experience is going to play a bigger and bigger role as time goes on. You MUST be able to show admissions committees that you know what you are getting into. You need to be able to demonstrate a knowledge of today's health care delivery system. This includes not only the technical and professional aspects of health care, but the social and political aspects as well.
The most important thing you should get out of your experiences is not simply that this procedure is done for this disease but rather how today's health care providers have to work within a very complex system that involves many other health care professions and providers, as well as many business aspects. You must be able to talk about and delineate how your personal value system and ethics integrates into the current health care delivery system. If you aren't able to do this in your personal statement and/or during an interview, you will most likely not be successful at getting into dental school.
There is no good answer to whether you should have tremendous experience at one place or a breadth of experience at many places with less time at each. The bottom line I think is this. You need consistent experience. If you can only commit 2 hours per week, but you do that over a year, that will be much better than 40 hours per week for a month. Time teaches. The more time you have, the better.
No. The dental schools do not care what your degree is in. What they care about is how well you did getting your degree, and how well you did academically on the required pre-requisite science courses. It does no good to get a biology degree, and then if you don't get into dental school, have a degree in something that you hate.
The question you should ask and answer is this. What would you be happy doing for a living if something happens and dental school is no longer in the picture? The answer to that should guide your decision as to what your degree should be in. Many students get biology degrees only because it is the quickest way to complete both a bachelor's degree and the pre-requisite science courses. However, and I want to stress this, DO NOT get a biology degree if you hate biology. It makes not sense. Get a degree in something you would be happy doing for a living in the event dental school does not work out for whatever reason.