I'm sorry if this question causes you pain, and I feel bad asking it, but I can't get poor Jack's confusion out of my head.
Were you able to save Mia's body or was it too late? If Jack is able to smell it, it will help him understand what is going on. I've heard about it happening a lot. He will still go through a grieving process, but animals can sense things like that and he will probably understand why she isn't there anymore.
If it isn't possible for him to do that, then he will probably come to terms with it eventually and stop looking for her.
Again, I'm so sorry if that question hurt you in any way.
I see that you are interested in getting another baby! that is wonderful that you are willing to help one in need. That is what Mia would want.

That may also help Jack to cope.
BIG hugs to you.