Magic BF weightloss is a myth, just a herbal pills, chinese tea, and food combining for weight loss is a myth. Weight loss happens when you use more energy than you eat, it's that simple. Various methods might help reduce food intake, or increase activity, but these only work if they are not negated on the other side of the balance (by reduced activity or increased food). BF will not make you lose weight if while you are BF, you are sitting on your bum eating cake! If you are eating and exercising as you normally would to maintain a steady weight, BF will probably lose you some. Being tired, sore and even thirsty makes a lot of people reach for snacks or sugary high calorie drinks.
Theoretically a proper balanced diet will have all the vitamins you need and supplements would be unnecessary. Personally I know I don't always eat properly (as per guidlines) and take BF supplements, but that is personal choice, not a medical recommendation. Even a badly nourished mother produces adequate BM.
I have no idea how the odd carton of formula would affect gut balance, it's all about pH and bacteria and stuff, but I'm not sure of the specifics. If your baby needs that formula, then she needs it, I wouldn't worry about it.
It seems like your problem could be with expressing rather than supply. Have you tried talking to a BF counsellor about how to express more effectively? I assume you are expressing while the baby is feeding off the other side, at a time of day when you know your breasts are full, with a decent pump and all that. I find having DS sleeping in my lap when I am expressing helps, because he is there for my let down, but I also get to express off both sides. Or if it's not every night then could you express on a daily basis and freeze portions to combine to make up a full feed?
I believe there is medication, but if you go do a Dr and ask for it, they will probably tell you not to bother and just feed a bottle as it's not worth the stress and hassle.If you were absolutely determined to EBF then you could push for it, but if not then you really don't need to worry about the odd formula feed.