yes this is looking challenging…
I would start out with a regular grid of cells. No cells are merged. Not yet.
Very fine grained. Where you will not touch ever * the top row or the bottom row (that will depend, if you plan to use a header row that repeats itself from text frame to text frame or from page to page)
1. Regular grid where the yellow row and the yellow column is your safe zone. Don't touch it. Or: Only touch it to split cells to create new rows or columns.
* you would remove this helper row and this helper column after absolutely finishing your work.
Or you would not remove it and use no fill color and no strokes for that.
2. Build the table step by step by merging cells.
If you want to add columns and rows, split the right cells in the safety zone and change the width of the cells there.
3. On with building:
If you never remove the safety zones during the building process, you always have full control over the whole table.
Thanks, you just solved my problem!