"I think she'll be able to predict with some certainty how she'll feel about the third one."
I'm not sure this is true. Also, what about how her baby feels?! I know my DS would most likely be terribly distraught if I was away for 9 days (he is if I'm away for a night). This would be an important factor in any decision for me. It's surprised me actually how many of these responses focus totally on how the OP might feel and don't even mention the baby's feelings as a consideration!
If the Bluesunday was saying "I know my one year old would cope with this, but should I go anyway" that would be different. But we're talking about a baby who isn't even born yet. And, as I'm sure you all know, - much as we might like to shape our babies ourselves - they have their own personalities. Bluesunday's baby might be very attached to her, and find this very difficult. Or s/he might not, but no one can say for sure now. I don;' see how anyone can say it the baby will be fine, because the truth is it's possible that it might not, we just don't know.
Also, while the OP is clearly an experienced mum, that doesn't mean she knows necessarily how she will feel about the trip in a year and a half. It's ten years since she last had a baby. I don't know about you but I'm already forgetting what I could and couldn't do with a newborn in tow, and that was only last year! Also, things might well be different this time round. The OP is ten years older and wiser, her body is ten years older too (sorry to mention it!). Her family dynamic is different to what it was then, at the very least because of the other children's ages and demands. All these things and more could affect her relationship with her baby and make it different to her relationship with her other children as babies.
Also the world has changed - for example the WHO now advises to BF until your baby is two. The OP might find she wants to BF beyond one year, and I feel you can't necessarily predict this until you have met your baby and started developing your relationship with them, even if you've had babies before!
I just think there are to many unknowns, and some of them would lead to situations which I really woudn't want to be in, if it was me - for example being forced to give up BFing before the time I felt was right for my child. This would be personal tragedy for me. Or leaving a baby behind which was very attached, and possibly damaging our relationship.
For me, it's just too much of a risk. There will be other trips she can go on if she really wants to, other ways to raise money for the NAS (an admirable aim BTW), and other things she can do which suit her family better.
I think it's unfair to encourage someone else to do something like this when in reality it could be a real wrench when it comes to it.
HTH