Hey guys, I have a problem about PPI and I already let my image editor app to set everything to 72 PPI since I don't know what is it, here's my full story, please read all to know my problem!

I make a deal with a image editor app company, Pixelmator, so they can give me free copy of the app to make my book, I promised I will use it to design all my book content and include a link to the app in my book.
My book is about origami tutorial and it will be in fixed layout graphic book form, there will be a lot of picture since I need to show step by step of the folding techniques. I using photo diagramming to show the steps so each steps is a real photo taken with my camera.
My plan is, took all photos, make a new Pixelmator file with the exact dimension of the picture, and I let the PPI set to 72 because I don't know what it is. Then import all photo to the file, so I can remove the background and retain the origami picture I want. Then I make a new Pixelmator file again(1875*2500pixel, 72ppi) to make a image as the page of the ebook, I design how the page looks and then import the edited origami photo to this page and make it smaller so every page will contain at least 6 photo, after few tweak, I export the page as a jpg/png(because I not sure which to use yet). Then I put all the jpg/png page to Pubcoder(ebook making tool), so every of the image is a page. Finally after all steps and cover part done, my book will publish in ePub3 and KF8 format. If my book really success and there is request to make physical book then I might do it.

So after you read my story, my questions are:
-is it a problem to use 72 ppi? I already done a half way and it will be too time consuming to go back again.
-I posted this question in other post and they say make sure every photo is around 300 ppi, does they mean the photo I took or the whole image page?
-Does rescale my book to around 300 ppi, why it look weird or pixelated?

if your answer is I must set higher than 72 ppi before I start, why if I rescale my book after it done to higher ppi, why it will make different? Since my book dimension is already a constant, 1875*2500pixel, changing the ppi only effect the book size in real world, why I can do it after my book done?

Thanks again for your patient!