"I was meditating on Atu III & was struck with the realization that the position of the Empress' arms & torso, while clearly indicating the Alchemical symbol of Salt"

That's a great observation that the Empress is showing the sign of alchemical salt. I took a closer look and the Priestess and the Emperor are likewise displaying the signs for mercury and sulphur, respectively. It's unmistakable.

But if so, how does the attribution of Priestess = Mercury, Empress = Salt and Emperor = Sulphur map onto the Tree in a coherent and meaningful process? The mapping might make sense if Binah=Salt, Chokmah=Sulphur and Kether=Mercury, i.e. correlating the three alchemical primes/gunas to the Supernals. But that's not how AC saw it.

"Is there any direct connection between the Alchemical symbol of Salt & the Sign of Isis Rejoicing?"

I think there is, yes. I haven't seen anything written to attest to this directly (either in Crowley's magical workbooks or elsewhere), but (1) I think you have the geometry more or less right and (2) since that's the sign of the 8=3 Grade attributed to Binah, there would have to be a connection of it to Salt.