The Emperor of Mankind

So is The Emperor of Mankind that gives "blessings and guidance" to the Imperium the real Emperor or is it some Warp entity created from the devotion and faith of The Imperium's massive population?

He's the real emperor who's being aditionaly empowered by the devotion and faith of the Imperium's massive population

Both.
See:

not OP, but did he actually call himself emperor, or is it a title bestowed upon him by others?

Almost everyone called him Emperor, it's probably the title given to him and the one he accepted. God Emperor of Mankind, however, is post Heresy and probably rustled that skeleton's jimmies something fierce before the flood of faith energy drugged him up.

Does anyone actually know that bastard's name?

Probably Malcador, but most everyone just knew him as "The Emperor," even during the unification wars (source: The Last Church, the earliest full story we've got in the timeline). He called himself Revalation during the same story, but that's because he was an evangelical fedora-tier atheist at the time.

Conan most likely.
His soul was a merger of thousands of shamans, but he was born into a normal family as a normal baby.

If there's enough people worshiping him and he gets a huge boost in power, can he "resurrect" himself?

Some folks say the Golden Throne itself keeps him from reviving

If the entire imperium isn't enough then no.

Or reincarnating.

Are the Grey Knights and/or the Custodians created withe The Emperor's genes?

Yes to both
but emprah didn't juice them up with power he stole from the chaos gods, like he did with the primarchs, so they aren't on the same power level

If The Emperor died would he be resurrected or replaced, or is The Imperium basically screwed?

Technically, it's keeping him alive, and as a Perpetual he should be able to resurrect.

The problem is that the brief time between his death and resurrection if he did die would be... well. Unpleasant.

Does anybody know this technique anymore? How has no Tzeentchian sorcerer or someone gotten the idea to fuse a bunch of psyker souls from their respective faction? The closest I can think is the Eldar, but their messiah requires they all die first.

>The closest I can think is the Eldar, but their messiah requires they all die first.
The shamans all had to die too. They made a suicide pact to make the Emperor.

Ynnead is literally Eldar Emperor

Yes but the shamans didn't constitute %100 of the human population. The Emperor didn't rebirth humanity at his conception, he simply came into being among the normies.

Conan. Oh better, what we know today as Conan is the heavily edited version of Emps in his wild youth.

What most people don't know is that the Emperor had a period in his life where he made really terrible movies.

There's a bit of resemblance, I guess

He's been alive since the beginning of human history. He's probably had thousands of names across the millenia.

It's also implied that this ability is what humans were invented for by the old ones, so there is no guarantee that it would work for the eldar.

Oh hai Malcador

Intentionally. He used his prescience to ply the future to see the reactions to every decision he made regarding the movie to create the pennacle of good-bad movies.

...

There's a fan theory based on hints dropped across a bunch of HH books that it's Logos. I doubt it's the case though, BL isn't that subtle.

His name is Gerald.