I'm so sorry for your loss. What an awful thing you are going through.
Almost every health professional, family member and friend has said words to the effect that his passing is probably a blessing in disguise due to his Downs Syndrome and having 3 other children under 10
Disgusting. I actually feel a bit sick. I'm amazed you haven't smacked any of them. I would have been very tempted. Absolutely appauling thing to think, never mind actually saying it to you. They should all be deeply ashamed of themselves.
A lot of people have a weird attitude to downsyndrome. For my most recent pregnancy I chose not to have a particular prenatal test that would have told us if the baby had down syndrome. I don't know what it would be called in England. Anyway, the amount of people who openly told me that I was being irresponsible was shocking. Most people thought that I had to find out out as soon as possible so that I could "do what needed to be done" at the earliest moment. Apparently it hadn't even crossed their minds that I wouldn't just automatically abort a baby with downsyndrome. When I told them I would want to continue the pregnancy regardless they mostly reacted by telling me that I was being very selfish. How could I allow someone like that to have to "suffer" through life.
It absolutely boils my blood. People can so cruel and nasty.
OP, in the nicest possible way, why the hell are you worrying about snapping at these people?
You ask how do I react to this?
My suggested response is to tell them to fuck off and get out of your face with their offensive views before you decide to cut them out of your life permanently.
Your baby died. Their reaction is horrifying. You owe these people nothing. Please just focus on looking after yourself right now and only surround yourself with people who support you. Don't waste another second worrying about how to react to people being offensive to you during this time.