either design extender or data bridge should be installed last, it really doesn't matter which of the 2 is the last one installed, as long as eCart is not the last one installed.
I suggested Uninstalling Design Extender and reinstalling it because it is the smaller one.
"Menus could not be updated. The extension is not being installed"
is a bug in the extension manager, and really the only fix for it is to rename the configuration folder.