god some people are being mean to you wd
i see where you're coming from - just give your friend the option of taking a step back if she wants to, but make it clear that you still do very much want her to be involved.
asaik, bridesmaids do have 'duties' - when i was my bf's bm i did as much as i could helping her get organised, going to fittings, picking her dress up, organising the hen party, etc etc - nothing life-changing but just stuff that helped her a little bit and freed her up.
and the getting ready together on the big day is probably the best part! and she might not feel fully part of it if her brand new baby is there, who would totally understandably be her main focus.
it doesn't sound like you are being a bridezilla, or even precious to me, just like a bridw who wants to do the right thing by her friend.
as for the dress - if it wasn't altered could you keep the receipt and return it if it turned out she didn't wear it after all? Or ebay, at a push?
good luck with whatever you decide. It's your day and you're entitled to have it exactly as you like. within reason