If Zoro has it, Sanji should have it too, in order to keep the rivality between them. And 3 users in the band would be too much in my opinion.
No, Shakky said she retaired "around 40 years ago", that is also what her Vivre Card says. She also said that she was being persecuted by Garp, and Garp doesn't persecute weak pirates, at that time he should be fighting the Rocks crew.
Also Oda very well implied that Shakky was in Rocks due to his response in a SBS I think. Shakky being in Rocks would also explain how she met Rayleigh, cause both crews were rivals.
So there are plenty more strong points than weak points in this theory,
It is a logia cause darkness is an ELEMENT, that is why Mochi Mochi no Mi is not a logia even though Kata can transform into Mochi, cause mochi is not a natural element, is artificial.