Hi @anon41087856, first of all thank you for all the amazing work that you are doing!
Believe me, you do and Lightroom has one. I’ll try to create one of these edits in Darktable with your suggestion and possibly make a video to illustrate the problems etc. If I succeed with polygon vector masks then weee!
Yes, but you cannot ever get a line that you want, only what the algo gives you. Again it’s time consuming to do trial and error by painting out different edges. If you had just 3-4 lines that you can position however you want and correct if not satisfied that would be much better. I’ll probably open a feature request for that one.
I don’t know, could be 
Yeah, I didn’t want to bother anyone but thank you xD I’ll send you the samples.