Tom, that was divine intervention for us your weary readers!!! You have amazing insights about your wife and your life but you do go on...
You know I care about you. If I could, I would host an 'intervention' for you. Your beautiful W is YOUR drug. You are addicted to her and the distractions of 'rescuing' her, it seems.
Stop getting your selfish 'fix' {Lake Tahoe, etc.} and consider that you have to sort things out before your little daughter starts mirroring your W. I know how protective you are of your children's mother. Please realize that in order to keep your kids on the right track and safe you need to interfere with her addiction differently. Your MIL could help. I understand she is part of the problem but it seems she understand the trouble from the inside and trusts you. She could organize the rehab and help you distance for as long as it takes for W to get sorted. Meanwhile, reclaim your kids and pull them away from this destructive yuk.