Sorry Spy, but I am definitely one who disagrees with you.
I think boundaries are key. I don't think you are being too restrictive in your requests.....it also sounds like you are including her in the decision on when it is apropriate to contact you.
I think although you need to be sensitive, as this is a terrible time for her...you also need to start a pattern for this relationship. A healthy pattern.
I agree that you should treat her as you would any other friend and return a call...but come on! If my friend was calling ME constantly and driving me crazy with worry, I'd have to tell her not to call so much too....It goes both ways.
you cannot and should not be the source of support and therapy for her. She needs to go elsewhere for that.
Right now you need to establish your role as your child's mother. You cannot do that properly if you are constantly worried about your child's birthmom.
You can be considerate, compassionate and loving towards your child's birthmom without it hurting your family.