I've sewn the most of it together by now. I will show some details on the sewing with the next post.
Off course there are some issues here and there.
First the front - I had to add darts to give it a more shaped look. The flat front was working only for the softer muslin fabric (so I learnt not to take a fabric with a too different stiffness for the muslin with no respect how ugly it is). I left the end of the darts open and created some pleats that way. I like them. What I did as a general mistake is that I took a bit too much away from the shoulder - so they look a bit too narrow. But I can live with this. What I don't like is the collar. It is definitely too wide and I am not sure if the rounding is good for the overall look. I will add a stand but I am not sure if a pointed collar wouldn't work better. What do you think?
Here you can see better that a narrower and a bit higher collar works better with the puff sleeve. The collar in the previous image is just hiding the detail.
The back: The darts still need some fitting - it still a bit wide. I didn't fuse the whole back as I did with the front. I think I am going to fuse the back to add some stiffness to it.
So all together there is no major drama till now. Only the narrow shoulders maybe.