1) Yeah. Extra Credit basically removes the concept of stacking from their (de)buffs entirely, so they can stack with themselves, with others, etc. Stacking free for all!
2) Nope. Cooldown and duration are describing the (de)buff "attack" itself, say they'll only do one of these attacks every 2.4s. The duration of the (de)buff is much longer, so it affects how fast it'll stack up.
3) Yeah, they can stack.