1. Word flows all content from top to bottom so any edits on page 1 will impact pagination on all subsequent pages. However you can insert a hard page break to force content to begin on a new page. This is easily done by pressing Ctrl-Enter. So if you expanded p1 so it now flows onto a second page which now includes content you wanted on a following page then you need to insert a hard page break at that location.
However, you should be using styles to format your content, and you can apply settings to a style which give you control over pagination. For instance if a particular style (like a chapter heading) should ALWAYS begin on a new page then that style should include the "Page Break Before" option in the Paragraph format. Lower level headings that don't have to begin on a new page should include "Keep with Next" to ensure they never fall on the bottom of a page and be separated from the following content.
2. Page counts are controlled at the Section level. If you have Section Breaks in your document then this can become complicated. At the simplest level, you can tell Word what page number a section begins on by going to Insert > Page Number > Format Page Numbers...
3. Assuming you used styles for your section headings, you can put a StyleRef field into the header to always show that section title in the header.
4. Fonts can have a effect on how a document stirs emotion. I don't recommend Arial or TNR for this application because I presume this is a work of passion. However, you said you have 400 pages already - therefore the bulk of the content needs to be set in a typeface that a) doesn't blow out the page count or b) strain the eyes and brain when reading large chunks.
I recommend you pick a 'romantic' typeface for your Section Headings but go with a more standard, highly readable typeface for the bulk of the text. You can find what others consider a 'Romantic' typeface on
this page . For the bulk of the text, I would recommend a less mainstream but still easily read typeface along the lines of 'Cormorant Infant', 'Garamond', 'Perpetua'. Look also at the name of the typeface as there is an opportunity to build on the sentiment - 'Perpetua' just sounds strong and everlasting whilst 'Comic Sans' is not going to be so forgiving.
Finally, I would recommend you do some exploration on
Charles Kenyon's Word site. He has collected a wealth of resources that will be a great help to you learning how to control Word. Feel free to come back here with specific questions but do spend some time looking around that site first because I think most of your questions will be answered there.