actually, they can spend it instantly, if they use the InstantDex again. Chained transactions will not be a problem (iirc)
not sure about this
while it might be possible, it becomes quite dangerous.
with bitcoins, you can stack up a bunch of tx that each link to each other and it should all resolve properly. With NXT, the other nodes wont accept the tx if you didnt have enough to cover it prior to the current block
so, no one size fits all solution
James