If you want to keep the armor you can do so but it won't dismantle into Bright Dust after Sept. 17. It will also become an Ornament for all Year 3 legendary armor, so you can wear it whenever you want to.
Can you answer this one what about the armor pieces from eververse that we don't have?
I don't have any information on if they will become available again. I'm sure the TWaB will talk more about it as we get closer to launch.