To clarify, the video feed to the healer is ONLY the video feed of their party frame/bars (I use grid). If you use that method you won't have to worry about changing targets on your tank.

Try making your second paladin ret and healing from one of the shaman. Shaman are better multibox healers (you can chain heal off the tank and top off the melee, for example, and they have earth shield and riptide to get some proactive healing going).

Click bars are actually a more complex way to heal and usually not something I'd recommend for wow (in other games you might not have a choice, like Lotro, if I'm remembering correctly from years and years ago).

IWT: I think you can get the behavior you want by disabling click to move on your healer and any other ranged, although I personally don't worry about keeping toons back out of combat except for bosses at levels 80+. Almost nothing cleaves anymore, which lets us be lazy now (the way I handle it then, btw, is to run in with my entire group and then strafe or back up the followers, and then IWT everyone and only melee toons will run back in. Binding your arrow keys to movement on only the slaves will probably help you with a lot of beginning movement issues).