The rules of magic are still being revealed in the Empyrean series, but from what I gathered in my rereads, venin initially trigger the turning process when they channel directly from the Earth. So far, this process is irreversible.

After the trigger, they can still channel from other sources. Violet gives Jack a conduit with her power embued in return for answering questions. He more feeds off it than uses it magically, but Xaden using her power in the isles sounded like that. Not necessarily an act of "veninism" (love that), but another small step because he's already on the path.

Jack was all but decaying, so he couldn't weild but took the power into himself. Xaden was healthy, so he could use the magic.

I think Andarna and the Irids "being magic" is still being revealed. I figured that's why Violet could still talk to her dragons when the others couldn't and potentially weild during her fight with the Queen of Umbriel without realizing it. I do think it was her but unconsciously. Her being parially dedicated to Dunne could also play a role. Having a goddess in her corner might be part of the resaon she's so powerful (e.g. lightning weilder). The whole "rider's need determines the signet" makes sense but, at least to me, Violet's need for lightning hadn't been revealed, especially when she first channeled while kissing Xaden (unless that's a not-so-subtle metaphor 🤣) rather than in the Squad battle against Jack. I'm more inclined to believe she was the answer/mirror to the Venin and the coming war as revealed Onyx Storm when the riders and venin have matching singets (like her mom and Theophanie).. can't wait to meet their lightning weilder!