Sorry, I know, this is 5 months late, but Adobe does not answer licence questions except to refer you to the licensing terms. I'm also pretty sure that this comes too late for you, but may help others in similar situations.
1) With an extended licence you are free to use the asset as you want (always, however, in the limits of the licence 😉 )
2) Why don't you create your art instead, when you want to change an artwork like this majorly? Changing the colours, extending lines etc does not sound like a major modification. For books there is always a section for credits. Add the title page credit there. If you do not have one, I suggest you add the credit line somewhere near your copyright message. Anyhow, crediting the art does not diminish your work. I've seen multiple books crediting this one or that one for the cover art and I never felt like: my gosh, they couldn't do this on their own... Please use credit where credit is due.
Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788 (Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).
Regards, Abambo Hard- and Software Engineer and Photographer.