1: We are a defensive class, most/best defensive boosts are belt-side
so belt first
2: it turns out crystals used on FA's hardly do any good (gossip?)
so imo it's better used on SA's so you can keep up on aggro and
survivability while grouping with too-high-dps's to get FA's
3: Given the fact they are in the Store as well, my guess is: yes
Crystals are nearly useless on a First Age LI. They were designed, by Turbine, to allow players to purchase NEAR First Age items from the store.
Casual player but want a First Age? Only 500TP at the store and you can get a crystal to install on your Second Age that makes it almost, very close to, as good as a First Age!
and yes I believe there will be new ones later....