First of all, my apologies for not wading through all the answers. I read only about half.
I am a mother of three also with my oldest being from a previous marriage. My husband, too, would not understand my need to go to the hospital or the need for my oldest to go. I have had countless experiences of my own (however none in the midst of an ICU situation).
First, I would say that yes, it is wise to ascertain whether your feelings for your ex are unresolved or truly good friendship.
If in ALL honesty, you are truly friends then I would say that it would be to your own benefit to go and see him but only under the circumstances that your husband goes with you and you take the child born of your ex.
The way I see it is this: if you do not go, you risk holding a grudge against your husband if your ex does not make it and you later feel guilt. You might even hold a grudge against your MIL for her attitude towards the situation. If you go without your husband, you risk your husband having wild fantasies about what really happened that are nothing similar to the truth and any number of people giving him their two cents worth about what they think your intentions were. If you do not take your son, you will regret giving him the opportunity for closure (if necessary and assuming they already have a relationship). If someone else takes your son, you risk feeling guilty for not providing your son the support and guidance he will need to deal with such a situation and also will feel at a disadvantage when he tries to describe his feelings or what he saw.
As for how to encourage your husband to go, I would say reminding him that you chose to marry HIM and bear two children with him shows your commitment to living your life with HIM alone. Asking him to go with you because you NEED his support and guidance for your oldest son shows that he has a place in your life AND your oldest. He is needed and an integral part of not only your life but your son's.
As far as the mother in law goes...I would say that she has some unresolved issues and nothing you can do will fix those. Her answer that "well..." only goes to show that your husband grew up in a very defensive environment and that she most likely cannot even comprehend the situation you have.
I wish you the best...be honest with yourself and those around you and make sure you have your own boundaries for what is and is not acceptable.
If your husband loves you, he may not appreciate your actions, but if you explain them, offer him the opportunity to participate and be completely upfront and honest with him and yourself, he will respect you for standing up for what you believe even if he does not agree. Be supportive of him, let him know you understand where he is coming from...how difficult and threatening it must be to be in his shoes and how grateful you are to have him as your husband. Acknowledging his feelings, encouraging him to be honest with you and showing him your unconditional love will goes miles further than anything else.