Can I make Apple Mail use "m" as the "mark unread" hotkey?
You can do the following:


Go to System Preferences > Keyboard > Shortcuts
click App Shortcuts and add a new shortcut using the "+" button
Select Mail from the list of your applications
As Menu Title, you have to put the exact name of the menu command you want to add. In your case, it should be "As unread"
Choose the shortcut you want
Just Add the modification


After these steps, just restart Mail and it's done. You can check on the menu bar to see that the shortcut has been modified.