Hm, tbh there's never a "safe" GPA net since it's a relatively holistic review of the applicant. Usually, GPA can offset a low DAT and vice versa. I would just try your best to definitely get your sGPA over a 3.0. There are actually GPA (and DAT) cutoffs depending on the school, meaning even if you apply, you will sometimes get filtered out just based on those numbers. I would aim to get your GPA as high as you can (hopefully your sGPA over 3.0 and cGPA over 3.3). Look into schools now to get a relative idea of which schools you want to apply to and their prerequisites on shadowing/classes/etc.
Yep, the scores are valid for 3 years for almost every school. If you're not going to retake the DAT (which you shouldn't necessarily have to, unless maybe you're 17 or below in any section which is the minimum cutoff for a good number of schools), I think it would be beneficial for you to look into post-bacc programs, especially if you have had an upward trend in your grades. I think it would help maximize that upward trend, which is why a lot of people who had lower GPAs do go on to do a post-bacc program, excel, and show that despite the initial academic struggle, they are actually able to take a rigorous science workload and do well. If you end up getting interviews, you could mention this struggle and trend upwards as well.
I'm not saying you have to take a post-bacc (especially since I don't know how strong other aspects of your app are), but I think it could help - it won't hurt at the very least, unless you don't do well (3.7+ to make a point I think). The only real "negative" side of doing a post-bacc is that you're taking at least another year to get to school, but applying early when your app isn't as good as it could be could also lead to not getting in, wasting money on apps, and needing to better yourself/re-apply the next year. I think a gap year isn't bad, especially if you purposely plan on taking it (as in you're not taking it simply because you didn't get in the previous year and now have to re-organize your thoughts and re-apply). This is my gap year, and though it kind of sucks being in limbo, I don't think I would have gotten in anywhere if I applied last year - I wasn't ready, didn't know enough about dental school, not enough shadowing, and personally would probably have been super stressed handling interviews and class at the same time.
I would definitely keep up your extracurriculars and your dental-related activities up until graduation if you can! At least most of them. And that's great that you got a job. It'll definitely look good and help you understand more about dentistry/the field/etc. If you do well, maybe you could keep in contact with the office and try to get a job there in your gap year. That could be another alternative to doing a post-bacc that improves your app with dental exposure and work experience, though that means that your GPA and DAT (assuming you don't retake) will be set in stone in an average-low range. In any case, try to make good relations with the dentist and get a rec from him/her 😉
Long reply, haha. I hope this helps you think about stuff at the very least!