Yes, 28 should be very achievable for anyone considering medical school, unless you have some serious standardized test taking issues (which some do; it's not a reflection on their ability to be a doctor).
I'm not sure if you're asking about BS/DO programs in general or just your specific one, but my advice if you're concerned about the undergrad portion would be to be sure you go to an undergrad where you'd be happy regardless of whether or not you end up in med school. Right now you may be 100% sure med school is for you, which is great, but college is a time where you're supposed to grow and explore and try new things. You're not supposed to be the same person in four years as you are now. In fact, if you were, there'd be a problem! You certainly may decide to continue to pursue medicine, but you may decide that another path is better for you. I really think that any undergrad you attend should be able to accommodate both possibilities. If that can happen with your undergrad dual admission portion, great; if not, I would seriously consider going the more traditional route.
It sounds like you're enthusiastic about the med school but tentative about the undergrad portion, which is quite a big a red flag for me.
Ultimately wherever you go, college will be what you make of it. Study hard, get good grades, and have fun!