Hey Mama, first, YOU CAN DO IT! Sending you hugs and encouragement. We've got some similar problems, babe, I've got 3 month old twins who also can only nurse with a shield (they were preemies, so they needed it at the start, and now I'm working on weaning, but it's not easy), and who also love their bottles of EBM.
I think a good solution for you would be to get an electric double pump so that when you do use bottles, it can still be BM. The best of both worlds, right?! Give them the best food, but via a method that is easier on you, so that you can give your boobs a break some of the time. The pump will also help you build up and maintain your supply, which is necessary for these little guys!
I find a pump absolutely essential. I will BF most of the time during the day -- they may get a bottle during the day if I pass them off to Grandma for a while or whatnot. Like you, I also started using only a bottle of EBM during the night, because they will just eat a bit, get it fast and easy, and go back to sleep.
My guys will also nurse forEVER. The 20-30 minute feed is a rarity. They are clearly not the most efficient nursers. Usually they'll eat for 45 minutes, all the while stopping and starting, taking their sweet time. I'll change them, then they're hungry again for more boob. Another 30-45 minutes. Fall asleep on me. If I move myself or try to move them, they wake up screaming, usually for more boob. This cycle goes on and on all day, so sometimes I just need to bottle it and give 'em a quick injection of milk, so they'll get food coma and pass out.
Also, you must get a twin size nursing pillow
. Absolutely required! This lets you do double football hold so easy. It's huge, but it's worth it!
To get the milk supply going, get some Mother's Milk tea
! It totally boosted my supply within one day. After a few more days I had even MORE of a boost. Worth a shot.
Lastly, for the cracked nipples: I got a cracked nippy a few weeks ago, and OMG it was awful. I went to the BF'ing clinic and my LC suggested Medela's Hydrogel pads
. They worked. I put one on, and overnight it was healed up. I kept using them for a week just to be sure. Ever since then, no more pain and no more cracking.