OP, put aside the wider debate about headscarfs. (Because that's not really what this is about, in my opinion.)
You have made a considered decision about how you live your life. Your MIL disagrees with your decision, and is not shy about voicing her disagreement. She even claims that your decision is responsible for making your husband ill (despite lack of exercise/poor diet being a far more likely candidate).
Well, to me it just looks like your MIL is being rather egotistical; she wants to be the Queen Bee and for you to do what she tells you. Nothing to do with religion, such women exist everywhere. Your choosing to do something differently to her is not seen as you choosing to do something for your own reasons; no, no, no, it is instead seen as a criticism of how she does it! Because, in her eyes, the world revolves around her and everything you do is all about her.
If it were not the headscarf, it would be something else. How you clean the bath, or what shops you use. Something. Anything.
Be aware that if you do knuckle under and wear headscarfs again, that will not be the end of it. Any other signs of difference will be picked up and criticised until you fall into line. After all (she will reason), it worked over the headscarf, didn't it?
Do you really want to make that rod for your back?
Oh, and as for her equally vocal friend - I think you said her daughters do not wear the headscarf? Take a leaf out of their book, and ignore the pair of them. You are the boss of you, nobody else.