In Word, my scene-break lines come out short and centered, while the Page preview shows them across the whole page. Which is right?

The preview. From version 1.9.9 the Word export draws the scene-break rule (three asterisks, dashes or underscores on their own line) across the full text width, with the same tight spacing as the preview. If you prefer a shorter ornament instead of a rule, type the characters you want on their own line and escape them with backslashes, for example \* \* \*, so they are printed as text.