Is is essentially because you disrupt automatic text flow. When you edit your discourse before the break, adding or removing stuff, you change the position of the “natural” break. Therefore, to make the manual break inconspicuous, you must move it the same as Writer would have done. Otherwise, either it occurs too early, leaving space at bottom of page, or too late, filling the current page but using only a few line on the next automatic one.
Thanks for pointing my incomplete statement. This “spill over” is not 100% reliable. I have designed a case where the result is not what people expect. This case is quite tricky and related to page parity with “blank” page insertion. I hesitate to qualify it a bug.
Scribus has its own file format but can probably export in a variety of others. However, I doubt it can correctly translate to ODF or DOCX, because formatting primitives are radically different. Scribus, and Quark XPress alike, are page-based. Pages are primary objects user allocates. Contents is then inserted in pages and can’t overflow the page. Office suites are flow-based. Your text is a stream which is spread over pages on demand. You only control indirectly the pages through addition of page breaks.
Considering this huge “philosophical” difference, I don’t think Scribus can export to text processing format.
Don’t, unless you use word “text box” as a generic term. In Writer text boxes are graphical “decorations” with very limited formatting possibilities. The “correct” way of handling your secondary flow in a comfortable way is to insert a text frame in your pages. However, you must insert manually a text frame in every page. The good news about it is you can link the text frames so that they all behave as a single one available for the secondary flow (text spills over to the next frame in the chain).
Consequently, you can start with this configuration which causes minimal fuss. In the beginning, size all your frames the same, approximately half the page size. While “tuning” your draft, you can change individually the size of each frame to “sync” both stories. Writer will reflow your text automatically.
I have no idea about the impact on performance. The more frames or tables, the more you may suffer some impact (usually unnoticeable with present-day computers). However linking frames over 100 pages is terra incognita for me.