1. Usually not, but you can expect that schools will look at two or three GPAs: your cumulative GPA, the GPA for your last 45 credits, and the GPA for the required prerequisites. If, say, your bad grades are from freshman year and you've done better since then, you haven't ruined your chances. But if you get a low grade in a pre-req the semester before you apply, that'll hurt you.
2. Anatomy and Physiology, Human Development and Abnormal Psychology. Those are the courses that pretty much every program seems to want. Statistics and Sociology are usually requirements, too. Some schools ask for other classes, but that depends on where you want to go. I've seen schools ask for Physics, Chemistry, an additional Psychology class, an advanced Expository Writing class, or some sort of 3D art class. If you haven't taken Anatomy and Physiology, be cautious: it may be more intuitive than Physics and Chemistry, but it can be even harder to get an A in.
3. This is a hard question to answer in any field. You sometimes hear about people with 2.5 GPAs getting into competitive programs, but you also sometimes hear about people with 3.8 GPAs getting rejected from lesser-known ones. In general, if you don't have above a 3.0 in your last 45 credit hours, I think you should make a point of targeting programs that get fewer applicants. With certain schools you want to be a 3.5 or higher. But that's just the impression I get, and I'm still only an applicant myself.
A doctorate in OT is usually for people who already have their Masters. I'm pretty sure it's better to just get a Masters at the school that fits you best, which, the odds are, probably doesn't offer an entry-level doctorate.
