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When I enter text with combining diacritics (like "ā́") the height of the cell increases and there seems to be no way to readjust it.

The problem is likely due to the graphemes having different typographic bounds. I suspect the ā́ with all its combining marks is simply taller, forcing the table cell/row height to expand. Nisus Writer won't allow the table cell to be smaller than the text it contains. The only way you might workaround this is to choose a different font, one that provides uniform text metrics for both the regular letter "a" and any variants you want to use, like ā́