Yes, cost is 50 points but Heat-1 is preferred. Not that if you get Heat-3 anyway it eventually costs 50 points...
About dominion and immortals I've never been sure of the way it works... Still having a more difficult to overcome dominion is an advantage for immortals
I don't think the heat-1 is a preference. It is a requirement, but it still causes the 5% income loss and related troubles. Just like with Abysia's humanbred theme and Jotun's Niefel theme: part of the cost is having to take an unpreferred temperature-scale.
You can try it if you don't believe me: take Marignon default with no scales and compare it to FoF Marignon with no scales aside from the required heat-1. You'll get 5% less income and 10% less supplies with the latter.
Fires of the Faith appear to keep the basic temperature preference of 0, though it requires Heat 1 (this will reduce their income, however). So I guess the theme effectively costs 50 points, since standard Marignon can increase Heat just like Fires of the Faith.
I really should refresh any thread *before* sending in my answer.