Greyed out Hide Column command
I know you said that you have unmerged all the cells, but if any of the cells in the column which you are trying to hide are merged with cells outside of that column, the Hide Column option is disabled, including header/footer rows.
You need to unmerge the cells to hide the column or hide all the columns which the merged cell(s) span. You can test this by adding a new column to the end, then selecting all the other columns and the Hide Columns option will be enabled.
Alternatively, Hide Column will be disabled if it is the only non-header column remaining in the table.