1) Yes, most people use an additive. There are so many products, I could not go through them all. West Marine actually has a nice write-up (in their catalog) on the pros and cons of each. I have used pretty much all of them with marginal results. The best one has probably been KO and CP, which is a biological treatment. However, it is expensive, has quirks (like you can only use CP to clean the toilet), and may have varying results on boats up north (temperature will slow bacterial growth).
I have recently plummed in a new system that uses a in-line strainer with cholorine and the blue teflon tabs (like you buy at walmart for home use). This keeps that awful smell from fumigating the boat when you pump the head, it is inexpensive, and so far seems to reduce the head smell to zero. ANother sailor I know has been using it for 15 years with no problems. Call the mfg of your hoses and tanks to make sure it is ok first. I did and it is fine.
2) 15 gallons. Manual head? You will be lucky to get by a week. Better put a tank minder on that thing if it does not have one now. First time you overfill into the vent hose, it will clog it and you will forever have a atrocious smell on your boat.
3) I cannot think of why the mineral oil will hurt. I do not know that it is neccessary???? Others may know better.
4) Yes. Once you have the prime, with the nozzle closed off, stick the hose into the pump out t-hull. Open the nozzle. It ususally has a sight glass that shows you when you have sucked air, but it also shows you water quality. Once you have pumped out, fill it with several gallons of water and repeat this process until you are pumping fairly clean water. THis gets a lot of the junk off the bottom and keeps a cleaner (and better smelling) tank. Then go below and add your favorite formula to your head and pump it through a bit.