This question had me thinking so I went down and did a quick test. With the Safeguard loaded and cammed, introducing slack line from the tag end as you described will not uncam the device. It remains locked until the release lever is pulled or the weight on the carabiner is removed. FWIW the rope in use here is 9mm Canyon Elite (Dano's camo version).
I also tried starting with the Safeguard uncammed and then applying tension to the bridge carabiner with a slack line above as shown in the picture. Pulling down on the bridge carabiner immediately cams the device over and locks it even with slack on the tag end as it pivots about the central rivet (the silver dot in the middle of the Safeguard) when loaded.
That said, in use I don't ever load the device (i.e. initially lean back onto my rappel rope) without tending the tag end with my brake hand first. Also, I would NEVER pull the lever without tending the free end with my brake hand. That would be a quick trip to the ground. However, I am confident given the geometry of the device that releasing the lever will always pull the cam closed resulting in a stop.
The issue would be, in the moment of free fall panic from an unintended brake hand release, is that you grab on harder to the lever to try to stop yourself rather than letting go. This is why the Grigri+ has the panic mode. When you pull the lever too hard (as in a panic situation) the lever disengages from the cam and allows the cam to close on its own. This is effectively exactly the same effect as letting go of the handle on the Safeguard in an emergency. The reason its safer is that you don't need to remember the only way to safely stop is to let go, the exact opposite reflex response your body is trying to enact while you're in freefall.
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