"I just think it's important to get the facts straight when it comes to the legality of stuff like that."
Honestly, it's no so much the facts as what the police overseeing these events are willing to tolerate. Illegal or not, they set the rules that people will be arrested by, and that's what really governs the protester behavior. Unless you feel like being a test case, anyway. The smart move is to relegate such tactics to friendly jurisdictions, as opposed to hostile regimes (friendly as in sympathetic to your cause, not in regards to gun laws necessarily; the antifa guys have clearly been allowed to avoid compliance with certain weapons and demonstration laws in the large cities most favorable to them)
Not that I have experience in this rather foolish endeavor (lots of ultimately worthwhile things are foolish) but it seems to me;
-A rifle is rather ill-suited for use in a close-packed crowd, especially considering that use would most likely be during a 'wave' attack or similar stampede. In Texas, pistols must remain holstered while carried, but rifles may be handheld so long as no brandishing occurs. Kind of a double standard IMO, at least as far as finger-above-trigger low ready, but that's how it is
-In a tight, busy crowd, the only way to secure something is to keep it close to your body, and within your field of vision at all times. Naturally that's where the rifle ends up, vertical to further reduce the amount protruding off your person. It's harder to hang a pistol or PDW in front of you without it being a horizontal-rig, in which case holstered-muzzling is a greater concern (at least visually, if not truly a safety issue).
-To hang a rifle vertical in front of you, a single-point sling seems like the only real option, with the barrel down. This has the side benefit of providing a convenient place to rest your hands (butt of the gun, or the grip) and making it nearly impossible to 'muzzle' anyone. While Texas, at least, does not consider a two-handed grip as you describe brandishing (even with a chambered gun), you open yourself & your fellow protesters to tremendous legal risk if you do not ensure that barrel is pointed nowhere that could be interpreted as threatening. It's rather telling how few, if any, photos there are of armed protesters of any stripe exhibiting even somewhat irresponsible muzzle behavior.
-The real purpose of these rifles, in light of the first point especially, is visual impact as opposed to practical self-defense use. "Pea-cocking" if you will as a deterrent measure. It is certainly effective on certain opponents, and you do have to admit the lack of incidents involving firearms to date at these rallies is pretty impressive considering practically every other implement has been used in violence so far (fists, bottles, clubs, knives, biohazard, chemical, automobiles, and even the internet via doxxing)