sea your plan makes sense to me, an NCT friend of mine did the same but they were able to put in a double bed so they all slept in the babies room, Barry is the same he cannot function if has not slept well or been fed and watered regularly, I think we put BB in her own room at 5 months another NCT lady put her baby in her own room from 6 weeks, its a difficult situation, could he not do a Friday or Saturday night so you can have a good rest in your own bed and then when mini is around 5 months maybe do a couple of nights together, I the the SIDs risk drops quite a bit at 4 months, its also hard to say if mini would have trouble with the transition, I think it depends if you co sleep or if he is in his cot, although all babies are different, we never co slept, the fear of smothering her stopped us but she always had her basket/cot, we put the cot up in our room so she was used to it then moved her into her room.
I used to be a perfectionist/control freak but have let the reins slip a bit, I did life coaching a while back and she made me realise that my world would not fall apart if something didn't happen just the way I wanted, it took time, one of the exercises a counsellor gave me years ago was not to make my bed for 2 nights, I have ALWAYS made the bed, so I did this very proud of myself, waited for my flatmate to come home and see, she took one look at it and said 'yeah this doesn't count as you have folded your pj's' so I had to do it again, honestly getting home and seeing the bed unmade gave me heart palpitations
and yes I still ALWAYS make the bed.
I suppose its about finding a balance and letting your standards drift. I can assure that you your world will continue as normal and others won't think less of you for letting things slide, you now have more important things to concentrate on, you need to cut yourself some slack and enjoy your little one, these are precious times :)
MIL on the otherhand will not be doing your blood pressure any good, WTAF about re organising cupboards
she might be trying to help but she needs to learn boundaries, you ask first surely, although BB constantly rearranges things for me 
I suppose my waffle is saying is try and let go of the unimportant things they really don't matter, only mini, you and Mr S matter and letting go of the control is quite nice once you get used to it it does take time to adjust