This is an awful dilemma, you’re really the piggy in the middle, although your husband might feel the same, if we knew what was going on in his head.
Yes, she is his mammy, the only one he can ever have. But you are his wife whom he has committed to for better or worse. So he has a commitment to you too.
As you rightly acknowledge, your mother-in-law must really miss her son. This is a real loss for her and I wonder if she ever really grieved it. She is now trying to control things in your life with him by the sounds of it.
Of course, this kind of interference and control will have very negative impacts on your relationship with your husband. She is his mother, not yours!
You do not mention if you have family of your own. If so, I suggest you link in with them for some support; even the balance a little.
It is quite serious for you to defer having a baby. The level of power your mother-in-law has in your life is very much unreasonable. You and your husband are both adults and it is not OK if this woman has this much control over your lives.
I would agree with you that it is not good timing now to fall pregnant as there is too much stress and tension in your lives. These issues need to be addressed before you take that step.