1. You need this item called Ethieric Light to ascend old weapons and armor which can stop during Prison of Elders, Iron Banner tank 3 & 5, etc. 2. What ever guns you find yourself using a lot, good exotics that you like. However I do recommend upgrading armor first to be a better match against higher enemies. 3. After you beat a Prison of Elders you get a treasure key automatically (1 per difficulty, per character, per week), you use this key to open a Chest below the air lock area in the Prison of Elders. Now the advantage of having more than one treasure key is to be able to run the Prison of Elders again on a difficulty you already beat it on that week and opening the same chest with your extra key. For your last question yea you should turn in all of your engrams when HOW drops, looks like you should open some of them them on your lower level characters to give them good gear.

To ascend: legendaries require etheric light, which drop from the nightfall, PoE and ToO; exotics require exotic shards to ascend. Do not dismantle your new gear. Ascend whatever weapons you think will be of the most benefit to you. I'm upgrading my exotics with my shards and my Fatebringer will be my first legendary weapon upgraded. The treasure keys are used to open the chest at the end of the PoE.