It sounds like you are doing pretty well, despite all that is going on. I am very happy that you are supportive of your D and her pregnancy. When I got pg (also at 17) my parents were anything but supportive and I really could have used it. Of course this isn't what you want for her, but in the long run children are a blessing and I know you know that. My "baby" is now 15, 6'4" and 220 lbs. And I always say he saved my life, I was going down a wrong path and becoming a mother changed all that.
I also think it's wonderful that your in-laws are so supportive of you. It sounds like your W is really making a mess of her life and like her Dad said, when she crashes, she's going to crash hard.
Lastly, I have been S from my H for a little over 3 months and I thought that was a super long time to be apart, but as I see you have been S for 8 months so maybe 3 months isn't really anything.
Keep your chin up.
"Everyone you meet has baggage. Find someone who loves you enough to help you unpack." ¤Formerly DelinquentGurl¤