I plan to grab them regardless just for the sake of collection. Though the light one might have to wait.
It looks like I might have misread the JP wiki again and my original post was correct on dates before my edits. Seems like the light Lilim/Lilith eidolon won't be in the very first union event as a reward. According to the JP wiki, it wasn't added until the reprint event which wasn't for a long time, which is why she's on the bottom of the list on the SSR eidolons page which is sorted by release. I'm not sure whether or not nutaku will give us her for our first or not. Though considering what they did recently with the rematch event shop, I'm rather worried.
And according to the JP wiki's event listing comments for the first union event, it had problems, hopefully nutaku doesn't give us that version but rather a updated version with possibly her as a reward.