Hi RocketQueen,
I would pick kid up after a few hours of party. Child sounds fairly safe getting there. It allows time with family, gives you space, honors court appointed weekend. I would agree to this with A while he is sober and have another family member who will be present aware of agreement and can vouch that this was the agreement - that yeah baby momma is popping in to get DD.
Now here's the trickier part of my advice. Be cordial and nonjudgemental. This is what his family does. They get together and drink. Wander in, smile and wave, ask to see cake smeared 1 yr old, depart with your kid. Whatever you need to do to set it up as a no drama kid pick up. Whether that means you bring a friend or wearing a hair shirt. This is your future - sharing a child with an alcoholic family - so identifying who might help you smooth out a lifetime of child pass offs will behoove you.