Licensing for self-published book

1) OK

2) You are allowed to make modifications.

3) As for the need for an extended licence, please refer to the licensing terms and the answer 1 here: https://community.adobe.com/t5/stock/license-and-usage-for-print-books-and-ebooks/m-p/10398218#M41363. At the end of the day, you need to know if the primary value of your book lies in the assets licensed from Adobe or not. (I simply assume that the print run of your book is less than 500k)

Don't expect anyone to give an actionable opinion here. The best Adobe staff will say here is that the terms of the licence cover all cases and that you should read them. This is correct, as your intended use may differ significantly from your description, and it is impossible to determine whether the images are an essential part of the product or only used for illustrative purposes. (see point 1 here https://community.adobe.com/t5/stock/license-and-usage-for-print-books-and-ebooks/m-p/10398218#M41363). If you are unsure of the licensing terms, you should ask a lawyer specialized in this matter.

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 on 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).