[WoW]Problem with macros and genarally making things work.

would simply adding a /targetlasttarget command at the end of your macro solve your problem?

I use a warrior main/priest alt and have a few macros along the lines of -

/stopcasting
/target Nimr
/cast Renew
/targetlasttarget

I guess it's active targetting, but it's effectively instant retargetting.

If you're hardcoding the healing then you can use the bracket modifiers and this wont change your target. An example would be:

/cast [target=Nimir] Renew;

Using bracket targets doesn't effect your clicked current target.

BUT

If you're using your main as the healer as you say, then mouseovers are the way to go:

/cast [noharm, target=mouseover] Renew;

No changing of targets, no clickes, no nothing. Move your mouse over the portrait or avatar of who you want to heal and hit a button.