I deleted some edges and a rest of the edge (point) will stay at the reflect plane. Merge edges does not work. Deleting this point will erase the whole edge and the reflected surfaces will merge. The only workflow I figured out is to delete the surface and insert a new one.
If you toggle SubD display it becomes more visible.
reflect.3dm (74.6 KB)
Or use Stitch command.
I can’t follow your thought. Stitch needs two elements. I’ve one point. Moving this point away from the reflect plane and stitching with the mirrored point creates again this point.