As for planet Vulcan, that name originates from the 19th century. It was hypothesized that a planet existed between the sun and Mercury, that wasn’t disproven until after Albert Einstein released his theory of relativity. So someone in the 50s would have been familiar with it. Memory is a weird thing and distorts more and more the further you are from the event. So if you asked adult George & Lorraine about that guy Calvin, they’d tell you he was much taller and more handsome than Marty, and clearly, he must have been a big underground influence in rock music, and maybe a male model ‘cos of the underwear brand? Darth Vader? Well, maybe he was a ghostwriter for Lucas. Everything can be explained, forgotten, or rationalized.
Well, 1, that’s not a “plot hole,” but 2, Pinhead never made some ironclad bargain with Kristy; he said “maybe.” At the beginning of the film series, the Cenobites are more amoral extradimensional entities rather than straight-up evil demons from hell. Pinhead may have felt he had no obligation to honor his dubious agreement. Also, he may have made the qualified bargain (again, he said “maybe”) believing Kristy herself was unlikely to be successful and would likely break the deal on her end when she inevitably failed.