: And what exactly was MB? Was he made from millions of cloned copies of a
: Forerunner? Or was he made some other way?

: And how well do stone age humans fit into the original concept of Prometheans
: as a collective of disparate parts? Knights, Rooks, Bishops, Pawns, etc.
: Why would brainwashed, technologically illiterate, mentally challenged
: cave men be the logical choice to implement that design?

Because they're not the product of a logically thinking mind at all. I used to be baffled by this and I've asked the same question a few times, but I think Silentium provided an adequate answer. While the Terminals in Halo 4 failed to communicate this, by the point of composing the humans, the Ur-Didact is nothing more than an indoctrinated puppet doing whatever the Gravemind wants him to.

I thought Silentium handled the issue surprisingly well, because it outright dropped the pretense that there was ever any viability or logic to the Knights. They're nothing but delusions contrived by a madman. On top of that there's an obvious irony at play there: the Didact's plan with the Composer is just a mechanical version of the Flood - no doubt the Gravemind's idea of a final insult.

While this solves the problem of the Didact's motivations (for the most part), it also presents us with another one. All this touting about us facing the Didact - who we know is supremely intelligent and noble, or actually an interesting antagonist - and then the Didact we end up with is but a sad, broken shadow of his former self (who also looks like a vampire for some reason) whose motivations are purely driven by his raving insanity. On top of that his minions consist not of his once formidable legions of warriors, but a horde of boring, emotionless robots.