My comments: Graduate school isn't something you just "do" because you're afraid you won't get a job and you're not sure what to do. It's a course of study that you undertake with the intent of BEING something. If you want to be a dentist and that's your goal, then you study for the DAT and go to dental school, but you do that only when you are relatively sure that you want to be a dentist. If you aren't yet sure what you want to do, find a job. There's no use in potentially wasting money on a career you aren't even sure you like, and if you haven't done any dentist shadowing, you don't know whether you like it or not.
Specific to your situations:
1. Most first jobs are mundane with mediocre pay. New college grads can't expect to make phenomenal money the first year they graduate. And there's nothing saying you have to stay there forever; look and see where the job takes you. It may take you to a more exciting and better-paid position 2-3 years down the line.
2. Do NOT get any kind of doctorate just for the "prestige." There's prestige because of the *work* required to get and maintain the doctorate, and if you don't really want to be a dentist, then you'll be miserable in it. There are also other ways to start your own business. But what do you mean people won't need dentists in the future? People will ALWAYS need dentists. We'll always have teeth!
3. Optometrists do sometimes start their own business, but don't assume that your connections will override the fact that you are not a competitive candidate. Just having the prerequisites is not enough - and this goes for the last one, too - there are thousands of students who want to go to professional school who have the prerequisites. What sets you apart is the passion and the experience; the assurance to the school that you know enough about the field that you are reasonably sure you want to do it and will not drop out of the program.
4. Physician assistant is a good career, but if you don't have the patient-contact experience you don't have "strong chances" of getting accepted yet. Patient contact experience is pretty much required for the vast majority of PA programs. But if you are considering this route, also consider getting a BSN/MSN in an accelerated program (like Yale's, Penn's, or Columbia's). They take 3 years, but nurse practitioners can practice independently, prescribe medication, and there are even more opportunities for them than PAs. NPs also move into administration and management very quickly - in 3-5 years, you can be a nurse manager or charge nurse for a floor. You could start your own business with collaborative agreements with a physician, work in a clinic or whatever. There are more NP programs at public universities which keep costs down; you could go back later and get your doctoral degree part-time if you wanted the "prestige" of having a doctoral degree (not worth it, IMO, but to each their own). And the pay is slightly better. At one of the better hospitals here in NYC, brand new NPs start at $96,000. I would say nationwide starting pay for NPs is around $75-95K, and you can go higher if you decide to go into hospital administration. Many nurses get MBAs or MSNs in nursing administration.
5. There aren't any prerequisite classes for MBA programs, but there is a work experience requirement. Like polarscribe said, even if you do get in (unlikely) without that, you won't get hired. Be aware that there is a glut of MBAs on the market; it's not a ticket to an automatic job.
You just started your senior year. Visit your university's career center and ask for some guidance and help. There may be more opportunities within biomedical engineering than you think.
If you want to travel around the world, consider joining the military. They will use your engineering degree. You are probably eligible for the Air Force bioenvironmental engineering job and aerospace physiologist.