using IS for 2 characters is just as easy as 5. Forget about modifiers in IS, ISboxer set them up for you.
The screen you show should be left as is to start with, untill you learn the meaning of the changes. i have used IS for a year and never change that screen,
Healing does not work the same a cast spell. as FTL set the mob as the target for you. Healing you need to set the target or WoW thinks you want to auto-heal yourself. You can either make a macro to heal or use a healing add-on with a click system.