Seeing as the UK monarch is also the head of the CofE, and that it seems to be a role that they all take very seriously, I would be amazed if Scientologists even tried to convert them. And if they did, I doubt very much whether William - or Harry - would be open to it.
But as PPs have said, Tom Cruise is an actor/director as well as a Scientologist, and the royals having tea with him is just some soft diplomacy to encourage future film-making in the UK.
Re: Jamaica. I am surprised by lots of people on social media saying how pleased the President was to see H&M, and comparing it to W&K's visit.
I mean, Harry may have "left" the Royal Family, but he still uses the titles, and so presumably is still very much linked to the past colonialisation etc. It's not like he has done very much to remove himself from the association!
And yes, if he is so concerned about the environment, why would you hop on a plane just to see a film. Bizarre.