Yeah, that sounds plausible. When we went a few years back, it was just as they were opening at around 11am, but it wouldn't surprise me if most of their business is late afternoon or early evening.
That may well be a viable option, then. Spend a few hours enjoying Fantasy Island until people are bored and moving on, those who want to do Mablethorpe can get the new creds up there late afternoon, and those headed towards Pleasurewood Hills who need the Bottons creds can potentially grab them en-route when passing back through Skegness on the way south.