illogical matters in Macbeth

YES, I am aware that "Macbeth" is a masterpiece one should not cavil at. And indeed, I do suspect the source of my problems rather in my head than in Shakespeare's, but whichever way, I have detected some incidents in the play that I am not fully satisfied with, as they elude my rigid concept of logic.

First, a most fundamental point: if I can somehow pull myself to get to terms with his decision to arrange himself for the success already fixed by fate, I just cannot see how Macbeth can be sure that after Duncan's death, it will not be Malcolm, who has just been proclaimed prince, following on the throne. For me, this would be the most plausible outcome, and as it would remove Macbeth even farther from kingship, I would strongly advise him not to provoke it...

Next, I just cannot see how Duncan's grooms can pray and talk, when they are drugged in so deep a sleep that "death and nature do contend about them". And what, Macbeth regrets not having responded "Amen" aloud?

Then, in Act 3, how can Macbeth have a nice chat with the bloodstained murderer reporting on Banquo's death in front of all the thanes (there is not even the kind instruction "aside"; the murderer just bluntly "enters" and begins his annunciation). ... Well, I had a few more wonderings while reading the play, but unfortunately didn't take them down, and don't remember right now... I will add more when my brain calls on me.

I am sorry to bother you with my logical derailments, or, if they should be not entirely so, my complete lack of academic ressources (I am a high school student). Also, for my grammatical and linguistic stumblings, that might add to the impression of a blunt axe (I am German).

I would be very grateful for some charitable help...

Luisa