I got the "Survive the night" achievement by sitting in one spot and fishing for about 50 minutes. Seriously, there's a lake right outside the town with giant bones you can stand on and be 98% safe while doing this! 98% safe because i have seen a group of grineer spawn there once.
Both and at the same time.
Warframe is technically "In Development", not exactly the Beta, but they release many Beta elements that may be changed in future (like Focus system), but you will see Beta mark for that content. Also they always are testing things and changing if they or community think, that things should be changed, even if they were fully released.
Long things short, PoE is fully released, but not fully finished yet, they are planning to add Weather mechanics, that will change Gameplay on that area, also they are adding new zones/mines/areas for PoE (they are closed now), also who knows what they will add there in future, so it's released, but still will be in development and will have more content in future, this is how DE works.