I want to focus a little more on the threat-level of the Endless, which doesn’t seem quite clear to me, yet.
I’m still not sure in which way the Endless are such a big threat 
“If” they were Precursors or of Precursor-origin…is that automatically “evil”?
I always thought they were pretty peaceful and “pro life” until the Foreunners turned against them - but basically only the Foreunners. So any beef there is should only be between Foreunners and Precursors. All the others pretty much don’t care.
The Mantle is also about “guarding” the Galaxy, a rather noble thing to do. Yes, the Foreunners perverted it by being too arrogant. However: The Mantle is NOT about ruling in fear.
So, when they were talking about “bigger threat than die Flood”, it feels like it’s always from a Foreunner point of view. To THEM and their beliefs, the Endless were a bigger threat - not to everyone else.
IF the Endless actually were objectively such a big threat to the galaxy, I don’t really see why the Foreunners did anything wrong by containing them. But isn’t the whole point of the story, to show how immoral the Foreunners were?
I can only imagine some sort of self-fulfilling prophecy, in which the Endless only became a threat because they were mistreated by the Foreunners and now want revenge. But then again…revenge against whom? Species that had nothing to do and probably don’t even care about any of this?
It’s all just about Foreunners vs. Endless, not a general threat. Otherwise, like mentioned above, the Foreunners did everything right, didn’t they?
The only option in which both concepts fit together (Endless were misjudged for no reason while also being a true threat) might have to do with the Flood, which might be able to harness the Endless’ ability.