Ok, but Scotland is not withdrawing "unilaterally" anyway: the government in Westminster specifically authorized this referendum. If there was some serious secession movement in the US along the lines of Scotland's, I don't think the federal government is going to send General Sherman in this time.
Totally disagree. Considering what flimsy pretext the US routinely invades countries since WWII, a local invasion would just make it that much more expedient. Look at how many well-enfranchised groups have made hay on the US war machine.
The constitution could be amended to explicitly allow this but I don't foresee that happening anytime soon. So yes, the US is firmly in #2.