Questions about eBook self-publishing for ibook, kindle on mac

hi mac people again!
I want to publish my own fixed layout origami tutorial ebook(no physical book, because I not sure how it work and heard about low royalty payment)

1) After a long research, I found that ePub3 is the best format I should use to publish to the respective platform, but months ago, I found an app for mac, not sure it is free or paid, is very great for making fixed layout ebook, even can export in both ePub3 and kindle kf8 format, the website I remember is very colourful, do you guys know?

2) And I see some post mentioned ePub3 is better than kf8, you can zoom smoothly on ePub3 but not kf8, is this correct?

3) Will eBook format affect sale? I see a post mentioned apple will only promote the book in it's ibook format, and kindle will do that on kf8 format, is this true?

4) Since I making origami tutorial book, which involve step by step guide, do I need to put video or interactive animation? will it just make the book more complex?

5) Do I need to buy the expensive ISBN for my book? will apple and kindle accept it without an ISBN? I originally want to publish it to google playbook too, but them seen like required ISBN, true?

6) I heard kindle device is very thin(16:9 ratio), I will make my book in standard 4:3 ratio, will kindle accept it? will my book page look tiny on kindle? should I care?

Thanks for ebook veteran helping me!

Have a nice day, cheer!!! 😀