You may be able to farm armor and weapons from encounters but I don't think you can farm keys. Also the limit you can hold is 5. Opening all 24 chests (which would require at least 5 people) removes the blight and reveals the Petra's Run wish.

[quote]I've been trying to get information on this for a while: since Last Wish is in rotation this week, can we farm ethereal keys? I just started playing last year and haven't gotten around to getting 1k yet. I have a (far-fetched) plan where I could farm like 25 keys, duo Queenswalk with my friend and then just open every chest at the end. If I don't find/receive an answer soon im just going to try it out for myself and report back to you guys lol[/quote] You should be able to since it is the weekly raid this week and it should function like any other loot drop.

You can only get three keys per week per character. I don’t believe this has changed since the addition of the raid rotator, All though could be wrong