Crux of the 17/18 connection are the morphing signs Jeffries shows Coop: from TP Owl Cave to 2 Lodges to the infinite number 8 (we saw the 8 before as motel room number). When Leland begs Coop to  'Find Laura'  he sets off but ends up in an endless limbo loop. With Leland dead and Laura resisting the dark side (barely, fearfully) Mother now inhibits Sarah with the Chalfonts and a seemingly  'normal family' forming a defence ring around her in the new arena for the eternal  battle between Good and Evi