Just to toss my hat in the pool and answer the first question:
As a commissioned officer, you can do a max of 30 years if you make captain. If you don't make admiral, you retire. As far as spyguy with 12 yrs prior, you'll only need 18 years as an officer to make the full 30, but since 12 of those years was enlisted, you can do longer than 30 (total).
If you have 9 years like jg5343, you can retire at 20 as an O-3E. Also, keep in mind if you make O-4, you need to do 3 years as an O-4 to get retirement pay for that pay grade. Under the high-3 program, each year you do after 20 is an extra 2.5% added to your retirement pay, maxing out at 75% for 30 years. So if you like what you're doing, staying past 20 ain't such a bad idea: Keep getting BAS/BAH and build a better retirement.
Yes, even if you're prior enlisted, you'll be USNR. Basically, all officers (minus LDO/CWO) are commissioned into the USNR. Why? I have my theories, but I won't bore you guys.
Another tidbit is that you're "safe" when you make O-4. For us non-prior folk, if you make O-4 and get passed twice for O-5, you'll be at the 17 year mark, and the navy will give you a 3-year extension to carry you to your 20 year mark.