It annoys me no end that the LMB is for the right hand and the RMB is for the left hand. Is there a mod or console command to swap them for combat only (i.e. not for menus, dialog, etc)?
Well, just go in your menu (Esc if on computer), Controls, and switch the buttons for Right Hand and Left Hand, and voilà
As Mikalichov says, it's easy to change in the menus on PC. I was irritated by this too and made the switch early in the game (around 20 hours, yes that's early).
This is only for combat. It doesn't effect menus, alchemy, enchanting, smithing or trading at all.
But there is not option to change the "preferred hand". For instance a single handed weapon always gravitates* to your right hand and spells/shields gravitate to the left hand. This happens even if you switch the mouse buttons and as far as I can see, there is no way to swap this.
If I'm wrong, PLEASE say, because I may be right-handed, but I'm index-fingered (that sounds bad).
*When I say gravitate I mean when using the favourite buttons 1-8.