Didn't Manchester Airport cope partly by using the long length of the platforms to park two trains in the same platform at the same time? That's an arrangement that I think is not ideal, partly because it's less convenient to passengers, partly because it restricts flexibility (the last train in HAS to be the first train out).
I do agree with you that in principle, two platforms can cope with quite a high frequency, but I'd be more cautious for a station like Llandudno, since it's not really a commuter destination so much as a holiday destination. So if you looked forward 20 years to a much better railway, what Llandudno would ideally be seeing would probably include a fair few long distance services from different destinations, with quite low frequencies on each individual route combining to give a reasonable frequency, and I'd imagine if those were being serviced properly they'd have longer turnaround times than commuter services would (things like cleaning the toilets etc.), plus the timetable wouldn't be completely regular, and longer distances gives more likelihood of delay to individual services - all of which would argue for more platforms to keep enough flexibility (Of course you could say the same of Manchester Airport!)