I'm not able to get the same result as you in regards to the default menu in IE8. I'm able to navigate to any of the sub level items without issue. I also tried this in compatibility mode and did not have any troubles.
When it comes to editing the menu I was able to edit the theme for the menu and adjust the colors to be just what I want them to be, then finish and the menu is updated to my new theme.
Please post back and include some steps that you are taking that are giving you these results so that we can try and look into this with you. If you have a link where the problem can be seen in IE8 that would be very helpful.
