I honestly and humbly apologise - you are totally correct - my macro is assist first then autoattack not AAtck then assist

Yep - re implied target - I use all my heals this way - to reiterate what you have said but in a different way:
- healer /assists the tank
- therefore, the healer will have the tanks mob targetted (and not the tank)
- if the tank takes a few hits, the healer casts a heal which will land on the mob's target - that target being the tank or whoever else is targetted at the time the spell was cast

All my heals are set this way as are my cures (except one) - it basically means that each cure/heal is targetless if you like - and also means I only have 1 of each heal/cure

There are cases when this approach may put you in strife though:
- tank gets stifled (so can't taunt), mob starts chewing on say the DPS, in this case I do have my 1 and only targetted macro being simply '/target tank /useability cure' to remove the stifle/stun so it can begin taunting again
- another example, tank pulls mob, mob takes a swipe at say the healer in passing knocking it down 50%, then retargets the tank - to heal the damaged char I just throw a group heal (as I can't target the specific char who is damaged without a lot of fiddling)
- above I made a point of including the statement 'at the time the spell was cast' - so an example, mob is targetting tank, I cast a heal intended for the tank as a top up, mid cast the mob chews on the DPS knocking down 50%, the heal I am casting will land on the tank not the DPS which is the new prioirty - to heal the DPS mid-cast I retap the heal spell and as I have /cancel_spellcast included in the macro it simply restarts casting but with the DPS as the recipient of the heal

Personally, I would not recommend using /target - this basically means that your slaves will have say the tank, and only the tank, targetted - yes, all dmg spells will land on the tanks target but all heals / cures anything and everything will land on the tank alone even when the mob is targetting another group member - you 'could' get around it by always casting group heals but they aren't the most efficient or fastest heals (both in cast and refresh) to be using all the time