1. First of all, ISldWorks::INewPart is obsolete method. Use ISldWorks::NewDocument or 'SldWorks::INewDocument2 method to create a new document.
2. You haven't posted codes for 'Centerrectangle' sub but I guess you're using ISketchManager::CreateCenterRectangle method to create the center rectangle. If this is the case, it will return an array of ISketchSegment which includes diagonals also. Use ISketchSegment::Select4 method to select these sketch segments. You can use ISketchSegment::ConstructionGeometry property to check whether the sketch segment is a construction line or not.
3. Remove theselines:
boolstatus = swExt.SelectByID2("Edge", swSelectType_e.swSelEDGES, 0, 0, 0, False, 0, Nothing, 0)
boolstatus = swExt.SelectByID2("Points", swSelectType_e.swSelSKETCHPOINTS, 0, 0, 0, False, 0, Nothing, 0)
I haven't tested it but it should work.