Your profile shows DPX 11. The most recent versions of Designer Pro let you indicate a starting page number. I don't think this is possible in v11.

Depending on your computer's memory and the number of images, you may be able to create your book in one file. I did a print book over 200 pages with many images in Designer Pro X15 and 16. It's a little clunky but it worked.

And I have done books in the past one file per chapter which I exported as PDF/X then combined the separate files into one file in Acrobat, though there are other PDF editing applications that can also do this.

I am not sure what you would want spreads and then single pages. Just create your book as spreads and add your single page content as you go along.

Sadly, and not for want of asking, at least a dozen times, you cannot set up separate left and right pages. You would think, but no.

As far as specs for margins, it's hard to say. For an e-book, not an issue.

I have published about a dozen paperback books with Amazon KDP (formerly CreateSpace) and they give you specs for inside margins. I have also used KDP to do e-books using their Kindle Create app. Basically, you import your PDF file and Kindle Create puts it together in e-book format and makes it available for sale/rental on Amazon.