I thought delay or reverb is only allowed when it doesn't stack with other sounds, so you can't let a sound ride out on the delay or reverb and play drums over it... You can however delay 'manually' and play drums in between. Or put a heavy sidechain on the drums.
I interpreted it as: one layer / one sound at a time for a whole track, regardless of the effects you choose to use.
To answer your questions as they've come up:
1) Single synth notes are fine, chords are not.
2) FX are totally fine, just not in a way that breaks rule 6. So like.. sidechain good. Delay bad.
3) Grifty and Birddrop wanted to layer an acapella over an otherwise monophonic beat. It sounds so nice that I'm allowing it. You have the option of including your acapella in the single-sound system, or layering it on top. Acapellas only though.