I know there has been at least one time in your life when you wanted to tell somebody something that you know that (a) they don't want to hear, and (b) would not be received well should you have decided to tell it to them anyway.
Well, this is another one of those times for me and since this person is Hubster's high school best friend's wife who lives three states away and with whom I am not more than an acquaintance. For that reason, I am posting my feelings and unsolicited advice here.
First, I am really sorry that your doctor thinks that you have an autoimmune disease. I cannot imagine the fear, uncertainty, and other emotions you are facing right now. I also cannot imagine dealing with my own uncertainty coupled with having one child with special needs and another child who is still an infant.
Here is my unsolicited advice, given not to be mean or petty or obnoxious or negative: for the immediate future, stop worrying about having a third child. Seriously. I have three that are each spaced roughly two years apart who, thank God, have no developmental or physical issues, and let me tell you, it's hard. It's difficult physically, but you know all about that. Imagine dealing with everything you're dealing with now, then add the stress of a newborn. On a good day, never mind when one of them is sick or really needs some extra TLC, my kids each want more attention that I am physically capable of giving them. Somebody, if not two somebodies, often must wait while I tend to the third. I go to bed most days feeling guilty that I can't clone myself or be in three places at once.
I guess what I'm trying to say here is this: I think you need to put your dreams of a larger family on hold right now, deal with the immediate issue (your health) first, and then see if having another child makes sense - not just financially (I'm sure he is making plenty and that money is not an issue), but physically and emotionally as well.
It seems to me that you're trying to force things to happen. I'm not trying to say that I know what God's purpose is for heaping you with so many obstacles, but maybe he's telling you to stop trying to make things happen and let Him do it for you.
If you read this, I'm sure you would be angry with me for presuming to know what's best for you. I don't blame you - I would feel the same way if our places were switched. But after reading your blog for two years, I am pretty sure that you and I are more similar than you could know. This is the advice that I would need to hear and accept and, as much as I would hate to hear it, would eventually realize is true.