To some what answer your question, when targeting the mob of say the tank and the mob is focusing on the tank you can use /ac Cure <tt> and it will cast cure on the mobs target, in this case the tank. So your macro would look something like this:

/macroicon Cure
/ac Cure" <tt>

I believe you can use <tt> in regards to say for example casting Aero on the tanks target, i have not messed with the following example, but it would seem logicial, I think.

/macroicon Aero
/ac Aero <tt>
/echo casting aero on <tt>. Focusing on <t> for cure.(where the single <t> will input players name)

As for targeting the closest mob, note this macro "should" lock on to your target and remain locked on until you slay said target, or cancel the lockon manually.. Have not test this maco either, but again should seem simple.

/macroicon Aero
/tenemy
/lockon
/ac Aero <t>