2) Keeping a mental picture of all the characters on stage at once, and using them in active ways.

Ug, this is sooo hard. Thank you for spelling out the issue so clearly! I was trying to frame it like, "having a full cast" but that's not quite right. It's making them part of the story that's hard, not having them there. Having them where you need them when you need them, not before, and not in a contrived way.
@__@

Great list! :D