If excel  doesn't provide what you want, then you might be out of luck.
AFAIK there is no user-accessible hover event in Excel.

I'm not sure how a tooltip would work for the contents of a combo  box
anyway. If you hovered over it, it would not know which data row to
display.

It is possible to trap the mouse  movement/position event and you could do
something with that - with significant effort. A Google search for "excel
hover event" found at least one hit for someone trying to do this.