OP here.
Thank you all! I went back and tried on my dress again. Then I compared it to the sample dress which the shop lady brought to the seamstress for me. I was wearing a wonderbra and spanx. The sample dress definitely fit better again. But it had already been sold to someone who was supposed to pick it up the next day, so I could not get it :(
They measured the cleavage of both dresses after I insisted, and they said it is the same length!?!?
The seamstress talked me out of shortening the shoulder straps because under the arms it will become to tight and she will have to put a seam on the side sleave which will have to be cut (she cannot take the whole lace off apparently). So I will go back next week...
We will definitely pin up the side sleeves a bit to lift them up.
I don't want that line between the boobs to show too much, so I might not wear a push-up or try and find another solution, maybe have some lace sewn in the cleavage.
It's really great to get all this advice from you!