Glued on??
Oh my
@crafter that's a bit of a nightmare. I've never had motifs glued on. I have however had dresses sold to brides by supposedly reputable salons who clearly thought more about their commission than anything else. This appears to be the case here.
If you can't remove the motifs (and you can't) then the only thing to do is either what you've done, or just take the darts, and sew right through all the layers, including the motifs. A part of you will die inside if you do that, but it'll get the fit right. Sometimes the fit is actually more important than the smooth running of design motifs, and more noticeable.
A thought - are you shortening the dress, at all? If so, could you just sew the darts as above, and then stitch a cut-out motif from the cut off hem over it, to hide the dart? I've done that before.
Despite our best efforts, sometimes things just can't be fixed "properly" and instead you need to do a "it'll do" job. I always let my client decide which option, if there's two or three "bad" options, to go for. I explain as clearly as I can that it's a compromise and that I'll do my best, but I can't work miracles.
Why would such an expensive dress have glued on motifs?