OK so about an hour after I posted this thread I realized it was a really easy fix and I essentially had what I already needed. I just combined the two auras I originally stated. When she summons spirits, it will announce that spirits are up in whatever chat channel you want. It will then create an 7.50 second bar timer to track (roughly) the duration of spirits chasing people as well as when for melee DPS to move preemptively away from melee. I've had some weird timing things with the WA however due to Lady's cast timers. I know for a fact the spirits are either an 11 or 12 sec cooldown, yet the '4 second' prewarning feels like it happens only a second or two before the next wave of spirits are summoned. Regardless, this is the best I have so far on this front.
https://pastebin.com/raw/xmUfA2V9
Now my next project is making an aura to see if I can see when spirits are targeting you. Even though you cannot target the Vengeful Shades, raid frames still display who has aggro on some unit which does correspond with the spirits. At least from what I have looked through so far, WA will not allow you to check threat on a specific unit other than your target, which you cannot target Shades. The only workaround I have right now is creating an aura that will check if you have threat on any unit and display a warning. I don't know to what level of threat the Shades have on a person, but since they were fixed they no longer have a dynamic aggro table and will stay stuck to one unit. I haven't been able to test this aura yet; this is one of the three I have made: one for red threat, one for orange, and one for yellow. Just dupe them and remove/change (to hide) the class loader if you're a non-Warrior.
https://pastebin.com/raw/J9PsqLFT
I'll post an update when I am able to test the above aura.