I would like to ask about licensing for ebook and print book cover.

You're asking plenty of questions:

Can I use the adobe (stock asset) as my primary image for book cover? 

Yes. That's why stock is there.

Does all your image royalty free?

All images are royalty free. You pay once for the licence, and you can use the asset in accordance to the terms, without further paying.

Does book covers with non-editorial image still considered as 'editorial works'? Do i need to credit the artist and adobe stock for non editorial images?

I would do so! I do not know, however, there is an obligation to do so. There is, IMHO, when you credit other contributors. Normally, with books, there is a page dedicated to some legal stuff and credits. This does not disturb the layout, does not cost a cent, so why not just giving credit, where credit is due.

Does the limitations of print runs with 500k for print books are also same for ebooks? I only need a cover for ebook, no print cover, does the limitation also 500k?

Yes. The print run is the combined run for print and e-book. But to be honest, any book with a run of 500k is a great success, and even at a minor royalty, you will be able to acquire the extended licence by then.

Can I modify the image(not in a bad way/not for politics etc)? I saw an image of a model I want to use for my sci-fi cover but I dont like the body posture or the dress she's wearing. Can I change the body with another image? Or i want to use a model for the clients horror cover, i only wanted the body posture but not the face, can i change the face with another face/scary face?

Yes.

Is a print/eBook cover an "editorial context" in the license?

No. You are allowed to modify the asset as you wish. But if you modify, you can't use an editorial asset, as it is not allowed to modify those.

Does print books/ebooks (not photobooks) considered as 'item for merchandise? Does your illustration images can be use for book cover as its primary value?

If the assets from Adobe stock are the primary value of the product, you should stop writing. If I did a book with stock illustrations to paint out, as we had as children, then yes, the primary value would be in the illustrations. But when I write a novel, the novel should be the primary value. If it's the cover, stop writing.

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).