william12 shared those numbers awhile back. He'd talked to a dev about it. william12 has also had a ton of toons in his instance macros and confirmed the numbers seemed about right. About 10 times harder to get an ultra rare inside an instance.
I'm not sure if raid zones have any bonus.
Descending Moon is TBL and GMM.
The EoK/RoS ultra rares are still popular because those were EoK raid item copies. Depending on the item, some have better heroic stats than ToV T1 gear, like for DPS toons working on +hDex. They still have some of the best clicky AA effects, particularly for 110 (RoS) toons. So like the Burnt Cloak is one level higher of damage shield AA than the ToV T1 tradeable one. The ones that give AC or defensive AA are nice and Skybarrage & pet ear are still in demand.