Thank you for your post.
This issue has been previously reported, and I have attached your entire post to the case.
In FileMaker 12, this issue appears somewhat improved. When I export the data to excel, the fill, text color, bold and italics formatting that I applied to the entire block of cells did retain formatting when I closed and re-opened the file--EXCEPT for the empty cells. The empty cells did not retain the fill color and clicking on them revealed that there was no other formatting specified for them.
If I then applied formats to these fields individually, the formatting was retained.