I joined when I was 17 and I must say I was terribly naive and gung-ho. If ordered to fire then I would have without thinking or considering the action as I hadn't received any training regarding UCMJ at that time (funny how training goes in that regard isn't it?). The 'current' me wouldn't have though and I doubt there are many soldiers that would either.
Coincidentally, I voluntarily enlisted when I was also 17, (had to get a waiver for being under 18, did you?). While I too received such training regarding the Uniform Code of Military Justice as you did, for some reason, the Billy Jack
movies sprang to mind when another 'boot' asked about having to follow "illegal orders".
That much I had factored in [re: Posse Comitatus Act], but the rumors part dealt with alleged questionnaires circulating regarding the use of deadly force against US civilians. That part is probably conspiracy rumor stemming from the actual urban warfare training itself. I have heard these rumors before, but only once to my memory.
You'd just mentioned a scenario, (looting), in which the use of deadly force by the U.S. military against U.S. citizens would be authorized. Another such instance where it is authorized is on 'sensitive' military installations, (in the CONUS and off-shore). Maybe it's just me however, there seems to be a none-too-subtle difference between Posse Comitatus being applied to looters and the same Act being applied to 'quell a domestic rebellion'.