Grav/Ice
Patron Pool access depends on whether or not you complete the first Patron Arc (with any of the 4 patrons) to get the badge. To do that you have to be of villain or rogue alignment. So as long as you meet those qualifiers you'll have access to them. As for your other questions, never played a Grav Dom so I can't really help you much.
I have a 50 grav/ice/ice. She's all perma and incarnate now. But before that it was a strong build. You set up your attacks by WHing into a pile, hit them with damage proc'd CF and Frost Breath. Run in and hit ISC. Hack away with swords and ice blasts til they're dead. Or just WH into your lap and hit ISC.
Please buff Ice Control.
Gravity seems to be generally disliked for doms (even with the coming tweaks) but I've started a grav/ice dominator as I think it works thematically for a water based villain I've got. Gravity reflecting the power of ocean tides and ice the ocean depths.
Has anyone tried this combination and persisted with it? Any hints/builds/suggestions?
On another thematic note, will I be able to access Leviathan Mastery from the patron pool?
Grateful and advice!