After I've connected to a table of data from Excel and selected "Import All Data," is there a place in Tableau that I can go to alter or adjust the data if I find an error or something that I didn't catch in the Excel data? Do I have to go back to Excel, fix it there, and then re-import it again?
To my knowledge, the easiest way would be to fix the data in Excel and re-import. Although it may theoretically be possible to edit data in Tableau (because it is essentially an XML file), I wouldn't attempt it except in an emergency.
The only exception is geospatial data in v7, where you can edit locations if they are not recognised by Tableau, and there is a built-in mechanism for it.