Since i am "famous" for wrong notes (or false notes), i hope to find some help here...
First of all: normally there are no such thing as false notes! Because music is still art - and you as the artist create the sounds ! Look at some of the "neoklassizistische" (<this is german, because i don´t know the english word for it) music...sometimes this kind of music sounds terrible, but it´s exactly the way it supposed to sound. If a painter wants to paint a green sky with yellow clouds, no one would say: such clouds don´t exist

When creating a ReMix i start with putting the original lead notes (the melody) in the piano roll. after also adding the original baseline notes, i want to give some more sounds to it...like a pad (or strings) that play the matching chords. And there is the (my) problem...i never went to a musician school or have some sort of musical education. I always made music just by ear. And that leads me to the following questions:
1) How can you tell, that the basic-tone of a song is (for example) Major C (C-Dur) ?
Is it the first note of the baseline? Or some kind of summary from the most played notes?
2) Can this change during the song ? (e.g. song starts with C-Maj but chorus plays in F-m)
I have searched the web for tutorials of this matter and found some, but they can´t answer these questions to my satisfaction.