How far away actually is this wedding?
You won't be able to book flights before your baby is actually born, and there's a risk that you won't be able to get a passport for them in time.
Honestly, you and your husband are being so nice trying to accommodate bridezilla but I think it is all going to end in tears and probably a lot of wasted money for you.
If I've understood your posts correctly, the couple have decided to get married in a location which is super inconvenient for everyone, involving international travel for all their friends and family, and not invite anyone's children. (Or are her sister's children invited? In which case there is absolutely no reason why your baby couldn't go too.) Did no one tell her that this is actually a really bad idea and that half her guests probably won't come?
I would almost feel sorry for her except for the fact that you've explained to her how difficult this is going to be for you to pull off and instead of giving her head a wobble and realising that it's already a huge thing that you're going to all this effort to try to attend her silly destination wedding and she should just let you bring your baby, she's doubling down.
That's on her. No one else.
You don't have to attend this wedding. You really, really, don't.
It doesn't matter if you've been friends since you were 4. She doesn't appreciate the Herculean efforts you are willing to go to to make this work, she won't even make the tiniest concession to make it more doable for you.
If she cared that much about your attendance she wouldn't be making it so difficult for you to attend.
So bugger her.
When she's spent an exorbitant sum of money on a fancy wedding which some of her nearest and dearest decline to go to because they just can't make it work financially, or because they can't find anyone to look after their children, maybe she'll reflect on what's really important in life. Or maybe she won't.
Either way, she's not as good a friend as you think she is.
Personally I think you'd be doing her a favour by saying no now, rather than saying no a month before the wedding when she'll be even more stressed than she is now. Give her time to get over it before the baby comes. You don't want to ruin your time with your brand new baby worrying about a silly wedding. That time goes so quickly and you don't want to waste it either running around like a blue arsed fly trying to get your baby a passport and organise this insanely complicated trip, or feeling sad because your friend isn't speaking to you because you're not going to her wedding.
Call her back and say, "I've decided that the baby and I come as a package. I don't want to spend thousands for DH and I to fly out to your wedding only to waste a dinner and have an empty seat next to me all day because my baby is banished from the wedding and DH is holed up in the hotel room. Getting over there with a tiny baby will already be a massive expense and a Herculean effort which you don't seem to appreciate, and I'm done bending over backwards trying to find solutions. Either DH, baby and I are all invited for the whole thing, or none of us are coming. Let me know what you decide."