Just the idea of regular troops being ordered to control a peaceful demonstration is way way out there.
The governor of the State where the demonstrations are being held will usually take control of the situation unless said governor requests federal troops.
I assume once regular army troops are deployed the governor who requested those troops will relinquish command of his/her NG troops over to the Regular Army commander.
The decision to issue live ammo is, I'm sure, made way up the chain of command, and whoever issues said order will be held accountable for it, so that order probably won't be made in haste.
I'm not saying it can't happen, as it sure would take a whole lot of brass to put live ammo out there. But once that live ammo is issued, for all intents and purpose, the authorization to shoot to kill on command goes with it, and every troop who gets issued those rounds understands that and that the situation they're about to face must be really bad, and therefore a shoot to kill order is distinctly possible, and they also realize that questioning the use of live ammo to shoot fellow citizens is an option that got rescinded the moment they got sworn in.
Shooting at a hole in the sky or a nearby statue when commanded to fire is an available option though.