Here is an answer I gave (after consulting with other "experts") to a similar question a year or so ago. This is, of course, a suggestion since there are no rules that deal specifically with this situation.
The matrix is more important than the headings. So, if the headings don't fit, the headings should be altered, not the matrices. That is, the matrices should be done with the normal spacing. Abbreviate or key the headings as necessary. Blank lines should be left above and below the matrix. I would not include the separation line under the headings because since there is a blank line, the "separation" is already handled. Also having both a separation line and a blank line under the headings just puts the headings one more line away from the column they identify. I'm assuming there is no problem with the row headings -- just leave one blank cell before the opening enlarged grouping symbol
With regard to your situation, I have attached an adjusted version of your braille. The column headings should go over only the actual column, not including the enlarged grouping symbols. If you put in the column separation line, a blank line is still required before the first line of the matrix. As you can see in the example, I have shown it both with and without separation lines.