Good question.
Text included in <div> need <br> for a break?
Yes, because as soon as you add a <div> (or any HTML), you are telling the editor "ok I am using HTML now so don't try to auto-format anything I add inside this html element". Line-breaks in HTML have no effect, so yes you would need to use <br> tag for line-break.
Because a simple "Return" then does not work on the frontpage as break
Correct. "Return" has no effect when used inside html, and line-breaks need to be written with <br>.
For text that is NOT wrapped inside custom html tags, line-breaks will occur automatically from the X3
Markdown editor.
In the future, we will probably add a html wysiwig editor ... but then of course, you cannot add custom html tags, unless someone (like you) edit the "src" first and add a custom html element. Html wysiwig editors might allow more features for visual editing, but it is easier to break something, as is not as flexible and clean as markdown.