Hi DesuDeus, here's one for the first one - the attached command will take a face selection, do a join of its edges to make a closed curve (or just duplicate the curve if the trimming boundary only had one curve in it like a circle trim curve for example), and then delete the face.

Copy the attached file into the \commands folder, then there will be a new command Named ConvertFaceToCurve which you can set up a shortcut key for.

It will not quite work properly on faces that have a "seam edge" like the body piece of a cylinder for example - the way that the seam edge has multiple connections makes it so it can't be joined into just one closed outline like on a simple flat face.

Hope it helps!

- Michael