Seamstress here!
Congratulations OP first and foremost. For your engagement and your baby.
Personally, if you came to me with this dress, I'd say change it. Obviously it's hard to tell without seeing the inside boning and structure of the dress. The saleswoman is right that corsets have more movement but this applies more to general weight gain and loss, as corsets as we all know cinch your waist in. Something that cannot be done safely and professionally if you're pregnant. Certainly not at 29 weeks pregnant. What can happen is instead of the corset going up in a V shape, depending on how you carry it will go up in more of a 0 shape or a U shape. Wider where you're carrying IYSWIM instead of that normal V corset shape.
I would change it. Sorry OP. Unless you carry teeny tiny, I don't think it's going to work and at the very least, you'll be really uncomfortable. Your best bet is to exchange and look for an empire dress, a maxi type dress or something with a much higher waist band.
What can also happen with your type of dress from a front / side point or view, when the corset is pulled tight from where the boning looks to be on that dress, it will pull in at certain points and not the others, so where the dress would naturally pull in on a non-pregnant woman, it won't on a pregnant woman and may try to compensate for this and pull in at the wrong places making it look a bit lop sided. It's really hard to explain.
Change the dress OP. It's your best bet. If someone came to me with this dress at 29 weeks pregnant I probably wouldn't even attempt it.
I've attached a picture of the type of dress that really suits pregnancy. But obviously it won't be your dream dress and different styles suit different people.