I typed a big reply to this and then the page wouldn't load when I tried to post.
Oh well, I guess I'll try again...
You probably don't actually have an infection... infections are quite rare and what most people think is an infection with piercings is really just irritation and other problems. It's no wonder you have swelling with all the sizes you skipped. If I were you I'd just stay at the 12G, you did the right thing by downsizing. Treat them like new piercings and do sea salt soaks once or twice a day until they feel better. Mix no more than 1/4 teaspoon sea salt per one cup hot water and soak your ears for 5-10 minutes. It's very important not to get too much salt in the water. Don't touch your ears or move your jewelry at all, which it already sounds like you're doing. Take some ibuprofen for the swelling and even ice them if necessary. After they're nice and healed, you can start massaging them with oil (vitamin E, jojoba, emu oil, and vegan versions of emu oil work best) to reduce scar tissue.
In the future, stretching should be done one size at a time, with about 6-8 weeks between sizes or until you can put in the next size without pain or resistance. Try to stay away form wearing tapers, and wearing acrylic or silicone in fresh stretches. Get yourself a pair of single-flare or no-flare plugs in glass, titanium, or high quality steel at each size. Good luck!