The undo/redo buttons are one of the only toolbars in Hammer you can't manually restore. I believe the only way to fix it is to edit Hammer's registry key. Be very careful when messing with the registry though, since it's very easy to corrupt it if you don't know what you're doing.
Regarding using keyboard shortcuts: the more you know off by heart, the better. You'll work way faster using keyboard shortcuts than clicking on all the on-screen icons.
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