As for S and depression, H has every right to know, he is the dad. I would let S know you will be informing his Dad, and go with the "we both love you and this affects all of us" route. Shutting H out of your kids' lives (or your son's) doesn't seem warranted. Your H has been pretty good so far with his S.
Christmas- It's early yet and I understand you don't want to leave your Mom hanging. maybe you could explain that it may be rather last minute? Us Moms are good that way when it comes to our kids . But I like your plan of dinner at your Mom's and making your own the next.
It's very early in your sitch. And I have a lot of hope for your relationship with H. You have to make it about you in the sense that you become the woman only a fool would leave (paraphrasing a vet here). When that happens, a funny thing happens as well- you realize that you are awesome and if H is in your life...great! If he isn't...kind of great as well!
What are some GAL activities you have planned, besides symphony?