A.B.
Your description here reminds me of myself, or, how I analyze situations I find myself in.
I offer this for your ponderings. May or may not apply to you or be beneficial.
A counselor once told me that the way I go about things (researching every option before making a decision) was healthy only up to a point, after which it simply became controlling. She helped me to see that even though intellectually, I know I can't control outcomes or other people, I have trouble internalizing that.
I understand wanting to spare your husband any more stress, but, imho, worrying about when/how to tell him is again, trying to control. I'm sure you'll be very supportive when you tell him, and will approach it with a "we can get through this together" mentality. If he does, as you fear, go into a deep state of depression over this, then that is more about him than you. Am I making any sense?
Also, chances are that there's never a "good" time to tell him, and, by waiting, it may only make things worse. Maybe you could think of it as, "However he takes this, we'll deal with the results, together." ?
And, too.......think of this......take care of you! You needn't deal with all of the stress of this by yourself........wouldn't it be better to join forces with your husband and share the burden? I certainly understand wanting to spare your family more stress, but sometimes, what is, just, is.
I thank you for allowing me to remind myself about situations like these. Trying to take care of everything myself is a long-standing habit with me. Might it be with you, too?
Best of luck, and I hope the health of your in-laws improves quickly.