Wow- this is like deja vu :)
When I was married to my first husband, we were in almost the same situation: we lived in the upstairs unit of a two-apartment home owned by his mother, who lived in the apartment downstairs. She was a little old Polish lady and had no problems at all walking right into our apartment whenever she wanted. When I was pregnant, I was still working as a grocery store manager and often didn't get home until after 11 PM. I would try and sleep through my morning sickness after my husband left for work.
My MIL would come right into the bedroom and ask me why I was still in bed and start picking up my husband's laundry! She also would come in when we weren't home and move things around- lol, I can laugh at it now, but at the time, it made me NUTS. She moved furniture, replaced a clock I had bought with another one she thought was 'nicer'... and after my son was born she always did just what she wanted with him and pretty much ignored what I asked her to do.
So where does that leave me, advice-wise? Here's what I can tell you:
Is there any other relative you can ask to assist you with? My SIL and her husband were aware that my MIL had some 'privacy issues' and we all had a family meeting about what was ok and what was not ok.
Also, -- and I am not trying to disregard what you're saying at all- but you are totally post-partum hormonal right now and EVERYTHING is getting on your nerves more than usual. I've been there and you can't ignore it, but I bet it is a factor.
Since you are living in her house, it's a little different. But I don't think asking her to leave your personal possessions alone is out of line. But there is a positive way to do this and a negative way.
Try saying " I've noticed that when I leave my purse on the hall table it's always moved. If I leave it on the dresser in our bedroom instead, will that work better for you? I just really need to be able to find it where I left it when I leave for work!" Say it NICELY- and unless your MIL is totally dense, she should get the message.
As far as the cleaning thing== honestly, honey, let that one go. At the very least, it is one less thing for you to worry about having to do. It is probably just HER habit or else she thinks she is helping you. Since you get along in other respects, it doesn't sound like it is meant as a criticism of you or your housekeeping habits.
If there are one or two SPECIFIC things that are really bothering you, again, just be NICE and say " I've noticed that you're always sorting the laundry in the basket. I usually wait until I take it to the washing machine to sort it. Would it be better if I do it upstairs instead?"
She will most likely either say no, of course however you do it is fine, or she will insist on doing it because she is trying to help.
It took me a long time to realize with my former MIL that nothing is as hurtful as thinking you are 'helping' and having that help turned away- especially, I think, for grandmas! Remember that even though it's making you crazy- in her mind she is trying to save you some work and make things easier for you!
As for not doing what you want with your son... ok, this one is so so frustrating and LOTS of new moms deal with it, whether they live with the grandparents or not, but it is much harder when you do. You may not want to hear this next bit of advice, but...
Let most of it go.
Your son is going to know for sure who 'mama' is, don't worry! Her not wanting to give him back to you is selfish, yes, but just put your stuff away, change out of your work clothes and come and hold out your arms and get him. If you need to just ignore her saying she just put him down for a nap or whatever, do that. Just take him and say " I had such a long day at work and I missed him so much ALL DAY. I really need to hold my baby right now."
Again, say it NICELY- but she will get the hint, trust me. Sometimes grabby grandmas just need a reminder here and there!
It's natural to feel jealous. My son is TEN and I hate the fact that I have to go to work everyday and am not home afterschool to be with him. I HATE it- I am totally jealous of moms who don't have to make that choice, lol! But you can't let jealousy sour your relationship with your in-laws or make you bitter.
As long as your son is safe in her care and she is not endangering him in anyway, just try not to worry about it. I know its hard, but think of it this way- grandparents are not around forever. Think of all the joy this relationship is bringing your MIL and how wonderful it is that your son will know her so well. Your son will be fine with a few things being different with grandma, so as long as it is not causing a huge issue with his health, etc. don't worry about it.
When I got divorced, *I* was the one who made sure that my son who was 4, still got over to see his 'Grandma Sweetie' as he has always called her. I put aside any personal frustration I had because I knew that spending some time with her provided stability for my son and I also knew that she was really afraid I would just cut them off from my son.
Amazingly enough, we now get along great! She is very fond of my second husband and now she and my 10 year old son play cards together, go to the movies on the Senior Bus and out for ice cream, and I get the wonderful experience of seeing my son 'looking out' for the grandma that took care of him as a baby. If you had asked me then if we could ever have this kind of relationship, I would have said NEVER.
Remember that even things that make you crazy can be done out of love and that you want these people in your son's life for a long long time. In her Girlfriend's Guide book, Vicki Iovine says " Fight the big fires and step over the small ones" and I've really taken that to heart.
Politely address the things are really important or REALLY a problem and let the others go with as much graciousness and love as you can. In another year you'll be in your own house and YOU can run the show!
Good luck and God bless!