Conversations about youtube videos already happen on youtube, so I'd much rather respond to your thoughts on the theories.
Personally, I believe humanity's final test is a test of character, not of might. Take humanity's first test at the Flood's hands as an example. Humanity had responsibility for the Flood's release, and they sacrificed a third of their total remaining population in an all-or-nothing attempt to stop the Flood. Their cure failed, but humanity passed the test, because they showed that they were willing to sacrifice themselves for the good of the rest of the galaxy. We also see this in humanity refusing to release the Primordial against the Forerunners, even when humanity was utterly defeated, reduced to the population of a single world, and at the mercy of the vengeful Forerunners. I believe humanity's final test will come when they're at a peak of power, not to test their power, but to test to see if humanity still shows the same self-sacrificial character, even when they're the undisputed strongest race in the galaxy.
At the current point in the timeline, no alien faction is able to stand against a united humanity (most can't even stand against just the Infinity). This would be an ideal time for them to be tested, but I don't think the Flood will administer the test. If the Flood emerges in force (with or without the AI under their command) the galaxy will fall as it did under the Forerunners' rule. Instead, I believe the Flood is using its influence to restrict access to technologies that it (or more specifically, the Precursors) don't believe the organic races should have at this time.
I expect that the emergence of the Flood on the Ark is tasked with curbing the Banished's power, and that it will not engage humanity as a whole (though it may fight the Spirit of Fire's crew, given that they're on the Ark, and the Flood doesn't want humanity to view it as an ally). I expect that Cortana's "Created" will be humanity's test, because even without the Flood, the Created are capable of defeating everyone else in the galaxy thanks to the Forerunner technology they've uncovered.
If I were to make a prediction, I would say that humanity's sacrifice required to pass this test will be the loss of the Infinity. I expect it will be sent to Maethrillian to wipe out Cortana at the source, to destroy or at least reset the Domain to purge her from it, and during this task, the Infinity will be destroyed.
I am interested to see what actually happens though.