Gwen, You are very new to this situation and I can understand your thoughts on not being able to share his 50th birthday w/him. You can rationalize w/someone who is in crisis. They don't think logically and trust me, you would not accomplish anything by trying to negotiate things w/him right now.
I'm sorry your D16 is having a difficult time and I think you are on the right track by offering stability, love and acceptance. It's very difficult for her right now and she will find her balance just knowing that you are there for her.
Cry, scream or do whatever you need to do to let off steam. You've got a lot of bottle up stress and anger that need to be let off. Come here if you want to vent. You have more common sense than you are giving yourself credit for. The coaster ride makes us second guess ourselves, but once you find your footing, things will fall into place and as you move through the grieving process, you'll discover your emotions may settle down too. Trust your gut. I think you've got a good handle on your situation and if you need to make decisions, sit on them for a bit and then decide.
Gwen, hang in there. I think you are doing well for what's been thrown at you.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.