Accessibility in Age of Empires II
You can.
Make sure you turn off 'Two Button Mouse' in the Options:
This will allow you to select and order all units.
I think most options should be available this way. You will probably have to make intensive use of the menu in the lower left corner.
Double-click selects all units of the same category in the visible area.
The most annoying thing I found in a short playthrough was the inability to unselect units: the best way to do this is to left-click and drag in an area without selectable units.