Right lady, backbone about to be provided, it's your home, your family and your rules, if they don't like it, they aren't going to be a big loss to your life. So, firstly, you tell them when they arrive that you can't be bothered going upstairs to feed your DC, if they have a problem with it, they are welcome to leave the room or leave the house altogether they can't tell you to go upstairs in your own home, refuse. If they have a problem with it, they'll stop coming round, which would be a good thing.
Secondly, re breastfeeding comments, they have already started the fight, go it's ok to be gloves off about FFing. (Normally not acceptable, until someone slags off breast feeding in my mind). I would go with "Well, yes, breastfeeding is a little more work than just sticking formula in a bottle but then, the health benefits are proven, and I really think it's more important that my child has the best start in life, and apparently breast fed babies have higher IQs, although that study isn't conclusive as it didn't take into consideration that mothers who breast feed also normally have higher IQs than those who don't."
Re the gym, are your ILs all Kate Moss thin? No? Well your answer is, "Gosh, I've just had a baby, what's your excuse for the extra pounds?"
Honestly, be rude as you like back, what's the worst the could happen?
Oh, and tell your DH he has to deal with his parents, if htey can't be nice to you, he should tell them to bugger off.