The problem is the wording. You're looking at it one direction, but your wording makes other look at it from another direction. You're really asking: how difficult of tasks should be used? This is where the DC comes from in 5E, rather than the treadmill often found in other editions and RPGs. The difficulty of any single task is the same, no matter who attempts it, or at what level.Okay, you are writing a module. Where do you put the DCs?
Or better yet, please read the example and give one fo those recommendations to the DM to calibrate it - do they calibrate that their superstar can often get the DC but still fail regularly, or that someone focusing so much shoudl pass almost all the time?
To answer the question you're really looking for, it depends on the purpose of the task. A group task should be really simple, assuming no modifier. A serious consequence for failure should assume no better than a Natural character, a minor consequence no better than Talented, and only a beneficial consequence assuming Focused. I never consider working together, except to determine if/how it can happen.
It is, sort of. Passive checks can be construed as the 5E form of taking the 10. While only Perception and Insight have a spot on the character sheets, and Investigation is only mentioned in a couple of feats, there's nothing that prevents the DM from using Passive checks with any skill or task. I use Passive "knowledge" checks for general information (seeking a specific bit of information requires an active roll).Take 10 isn’t a thing in 5e.