For the "mellow" stuff, it all seems waaaayyyy too simplistic to me. The vocals are the main part and pretty much carry the melody of the song. An easy way to write for this would be to come up with whatever lyrics you want (writing the melody as you go) and then just base simple chords around whatever key you are singing in or note the lyric is on. This is all just my guess though. Doesn't seem overly difficult.