do i have to order roundtrip ticket for ngong ping pick cable car to go to disneyland?

No, you don’t have to order a round trip ticket for the cable car. It is fine to ride for a single way, or you can get there by bus.

It is possible to visit the Buddha only and then head for Disneyland, but the time may be quite tight for you in Disneyland and you may probably miss some entertainment items. In this case, you are advised to visit these two places respectively.