1. Let's think about timeline and priorities. Most people want to get into a categorical program as soon as possible, so I'm going to let you in on what I think the best strategy would be for a current fourth year with a low class rank and low CBSE score to do such a thing. You can extrapolate this to anyone with any score and any class rank really.
YMMV and everyone has different strengths and weaknesses that may change their strategy. I'm just dishing out advice using an imaginary set of rules that I observed through my own application cycle and observing the ranking process at my own program.
Here are the rules:
A. CBSE score is the most important part of your application. If you can get a score of 80+, you will grab any evaluator's attention, and you will almost be guaranteed to match somewhere if you are not otherwise a total dip****. 70+ will get you interviews very reliably, and you will most likely match. If you are in the 60's or lower you will need something else (research, personal connections) to define you and these people will be less certain they will match at their desired destinations if even at all.
B. The only extracurriculars that matter are research (On any subject, if it is meaningful) and impressive positions such as class president or some kind of volunteering director (but you must have done something meaningful with this position. During interviews you will be asked what you did with that position and if it's just "winning an election," nobody really cares. I'm sure people will read this who had other stuff on their resumes and will dispute this, but I'm making generalities for simplification. I would even go as far as to say that only research gets you an interview, and the other positions only help you during the interview itself.
C. The intern year gives you a leg-up on an equal applicant who has not done an intern year (again, generalities), and also makes your categorical intern year smoother. If you're an absolute superstar intern, you will get a leg-up at matching at the home institution. Those are the only advantages. You can also blow it at the home institution if you're crappy. Taking the CBSE during a non-categorical intern year will most likely severely hurt your score.
D. Any experience that has nothing to do with OMFS, like, let's say a year of GP or a GPR, does not hurt you as long as you can explain why you did it. If you needed to pay off some loans or you were unsure of what you wanted to do that's very understandable, as long as you are certain that you want to do OMFS when you apply and you make that clear in the interview room. In fact, someone who has done a year of GP has held down a job (most applicants' first full-time job is their residency), knows the importance of speed, patient experience, and coding, and has a full year of real-world patient interaction under his or her belt. If anyone questions your decisions in an interview, you bring up those things and boom, you just turned a supposed weakness into a strength.
E. Applying for a categorical position does not preclude you from later applying for a non-categorical position if you do not match. In fact, this is the sequence taken by most non-categoricals.
F. If you have not done any externships during dental school, you can actually do them after you graduate. All you need is 4-5 weeks of externships to match the average applicant. You won't be paid and would have to take time off of work, but theoretically you are still saving money and time over a non-categorical year.
You posited your question as if you are obligated to take an entire year until you apply. I'm going to assume you've already done enough externships to make things simple. This year, the CBSE is in mid-August. If your dental school class graduates in late May or early June, you then have 2.5 months between graduation and the CBSE. This is a pretty solid amount of time, although I usually recommend 4 months of hardcore CBSE studying. You can also start while you're still in dental school to get those full 4 months. You can start right now and have 9 months! (remember, getting an 80 will almost guarantee that you match). In summary, you have plenty of time to get a good score if you manage your time well. When you graduate and start your GP job, just make studying a priority and take it very seriously. If you don't match this cycle, then you can apply for an intern year, take some time off from your GP job before you start it, and try to boost your score again.
2. See Rule A above. Each year something like 8 people will get 90+, 20 people 80+. 70+ is becoming a lot more common. I'm sure there are more accurate statistics out there but it feels like at least 50% of applicants get 69 and below.
3. It would be irresponsible for me to answer this. I simply don't know enough about it, and every program changes so much year-to-year depending on the seniors. The best one in my opinion is at my home institution, but I don't want to compromise my identity, so I apologize for that.
