I am sure that everyone who reads your post will feel for you. I cannot imagine what it must be like coping in your circumstances. You mustn't though let it ruin your life or your marriage. Life does go on and although what has happened will always be with you, there will be good times ahead, though at this point I'm sure you cannot see when that will be.
As I made clear in my first post on this topic there is still a lot of research going on.
98 new research papers listed on this topic in the last 180 days at
PUBMED so although there is still a lot to know it's good to know so many folks are working on the problem. It's unfortunate that so much of the research budget for health matters is driven by the profit motive rather than preventative medicine and strategies to improve life.
This paper
Developmental changes in infant heart rate responses to head-up tilting. Looks like it may have significance in your case. The conclusion being
Newborns react like adults, mounting sustained increases in heart rate in response to head-up tilting, but at 2-4 mo of age sustained heart rate responses are no longer significant.
But I'm not sure how one can be sure what is the right speed to tilt a four month old as by that time the new parent is becoming somewhat more confident in handling the baby as the initial nervousness will be wearing off.