in all honesty, i wouldnt' go if i couldn't take the baby - it's not really convenient (and phooey to your DP about not taking DD to a wedding! pffffffft! grumpy bum!). it's going to put undue pressure on you to build up adequate supply for her, and for her to be able to take EBM from someone else at that age. at 3 months they face a growth spurt, wonder week - and all they really want is mummy to be there. how would you cope being so far away if DP rang you in distress cos the baby just won't settle for him?

personally i couldn't do it

i would contact your friend and say you're up in the air, but given it will be so soon after having DD, chances are you won't be able to come as you can't come as a family (financially) and you'd have to leave DD at home which would be stressful for everyone

oh, and another pfffffffffft to DP - does he not think you're smart enough to check the installation of the car seat?

ETA - it sounds like he is trying to ask you not to go for whatever reason (financial is my guess seeing you're trying to sort finances for your house) - but doesn't want to say no we can't swing it, and is making excuses.