If border edges still arent showing goto your custom polygon display menu with your object se;lected and you can turn on border edges through that set the size to around 3 for noticeable edges but are easy to work with I have attached an image of where you have border edges make sure all these verts are snapped together and then merge them also
If that doesn't work try a quick POLYGON -> CLEANUP if yor settings set to select and cleanup and non manifold geometry if it selects anything just deselect it and leave it and it should be fine