Well, I can throw my 2 cents into this.
Regarding oil, there is lots of options and each person will find a favorite. I recommend getting small amounts at first until you find something that really works with your skin. 8 or 16 oz (or equiv in metric) are good starting volumes.
I personally have tried several and coconut was one of them. I found coconut to be extremely greasey and very slow to absorb. Therefore, it made handling the domes, pump and anything else very messy and difficult. It felt good and helped but the logistics of using it just got to be too much trouble. When I tried safflower oil, it is heavier and slow absorber too and caused acme for me. I love cocoa butter (raw) and almond oil personally. Very light and much faster absorbers and easy to clean up. (BTW, flaxseed is a good one too -- and nice on salads too
)
Still, I recommend you try many oils (and heavy lotions too) as you'll never find that wonderful mix of things until you experiment. I even found a Wild Yam cream that did really well too without the oils.
Now the subject of dome sizes. I've used the small and I had a wide base to start with. IMHO, the base isn't as important as lob development (the nipple and tissue surrounding it). You have this already and should at a minimum be a medium IMHO. The sizing from S to M will seem like a fair jump for your frame but it will be more comfortable.
As for the L, I'm reluctant to recommend it as a jump from S to L on a 28" (if I recall) underbust will present some serious challenges initially. But again as James points out, you'll be going there and it is sized better for the 28-30" chest. If you don't mind the challenges and willing to wait a bit longer to "see" growth then it will save you some money and the process of changing cups.
DJ