I completely understand your need to go home, that is what we do when we feel depressed and need support.
Your MIL perhaps does not understand, it is not just about seeing your parents, it is about being in your home country and being in familiar surroundings.
Bringing the cousin in America into the discussion is not helpful. The cousin would probably love to see her parent more often but cannot. We are so lucky to be living in a world of cheap airfare. I see my parent about every 6 to 8 weeks, sometimes they visit us and we go over at least once a year.
And if one more person says, "In my day, folk emigrated to Australia and you never saw them again" I will SCREAM!
As to distrupting your DS's routine, I think that a happy mum is more useful to him than a strict routine.
Your DH is a man, he can cope without you for a while.
Have you had any treatment for Post Natal Depression? I had this after my DS birth and spent a couple of weeks at MILs then at my parents and let them spoil me. That, combined with the happy pills got me back on my feet again.