OP your post reminds me of the first I ever posted on Mumsnet 2.5 years ago... you are not alone.
AttilaMeerkat is great for advice, listen to her; she has been of great support to me under the guises of several different usernames in MN. On the other hand, your situation is yours alone and it's up to you on how you decide to react to the advice you're given on here. Although there are similarities between your situation and others your situation is still unique to you.
My MIL was just like yours, came to 'help' when DC was 6 weeks old and at first she was great, them DH went back to work and she became intolerable. She rearranged my kitchen, criticised my parenting, said that I was 'making a rod for my own back' by picking DC up when she cried, gave me no space whatsoever. DH knew how his mother could behave but still expected me to 'tolerate' her as he has done all his life. At first, my DH responded like yours and appeared disappointed in me for feeling the way I did. Atleast you stood up for yourself, it took a long long time for me to do that.
I ended up with PND at the hands of MIL because I felt like my DC belonged to her. It was a very strange feeling. I felt like her babysitter and MIL felt like her 'real' mother. That is because she ground me down so much and questioned so much that she made me feel like an incapable child. That is not a healthy place to be with a young baby to take care of. Your mental health is your second priority after your baby. Prioritise it.
Tell your DH, she makes me feel x,y,z and I need to prioritise my own well being which means space from MIL for a while. He needs to understand this. Like you, my mother is not maternal and I'm so used to being independent and self-reliant, this sort of selfish indulgence from MIL draped in good intentions is completely foreign to you and you will feel sick to your stomach.
We are still having problems with MIL. DH is much more supportive than he used to be but still pleads with me to be more 'tolerant.' I am me, she has crossed my boundaries too many times so I can't be. DH is getting used to this and I believe currently working out whether or not he can be with the 'real me' at all. That's his shit to deal with, not my problem.
Just give yourself whatever you need to protect your sanity, you are the one who has just had a baby, you are so important right now.
My DC is now 2.5 and still, my MIL makes me feel like she belongs to her. Like you say, an extention of DH, I feel like a surrogate at times, like I've had a baby FOR her. Of course, she never says these things bit her behaviour makes me feel it very very strongly. I think she had me down as a weak woman who was merely going to.pass my baby over to her 'the expert' who has a history in childcare and she was so so wrong. You are the expert in your own life and your own child. Make sure your DH and MIL knows that. Good luck. It's a long road but keep on sticking to your guns.