Sorry for the slow response and thank you for your intrest.
I don't know who the God Emperor is but yes D has fought stated universal beings many times such as the destroyer, lord Vlad(who killled the destoryer or one of them), Lord Vacua, Gilzen, Elder God, etc. Cuts from his blade ignores durabiity and attacks on a conceptual level beyond the physical. So yea I agree beings on this level can easily be cut down by him and have.
Not necessarily. Left hand can help with any and all that plus he is very resistant to those attacks. The times he was manipulated in his sleep he went to sleep on his own he wasn't forced to. Also he has experience and has resisted sleep manipulation from LUI.
The Nobiity can induce sleep, all of them have that ability along with dizzy, and telepathic abilities like forget he really has high resistance to. Disorientation from the maze like dimensions he has been sent to is actually a higher form than some biological attack of dizzy such as when D wen through Temple made in a God's Image
, King Minos's Labyrinth, fought Sigma, or went through Moebius Space. Those are all areas that would induce a dizzy or disorientating power that no normal human could survive just being there.
Not to mention left hand also has those abilities to induce sleep,dizzy, and forget along with higher forms of those kinds of ailments.
Those kind of attacks are low level to D so much so the books rarely have some one attempt it because they are absurd to even try.
is it true that the Big Bang Accelarator has the capacity to destroy a universe? in the books it stated that it has the same energy that created the universe. THE BIG BANG THEORY. what is the missing part of the accelarator?.