You said you're using an FTL system.
FTL is for your assist; without focus/target/leader set, your team will know where a command has originated based on the modifier that is sent with the command.

You're attempting a simple set of macros, which will not require assist to execute.

Macro 1, Keybind: NumPad 1
(Everyone) /cast [target=ToonA] Heal Spell

Macro 2, Keybind: NumPad 2
(Everyone) /cast [target=ToonB] Heal Spell

Etc.



The problem with a Leader based assist.
Or in this case, a leader based heal, is that it is going to break down in battlegrounds (or LFR raids, unless the leader is willing to move all five of your toons into the same group).

In battlegrounds, only the BG leader (or those given assist) can rearrange groups, and even then they can only move players from their own server.
Your characters will often be in 2-3 different groups, when boxing five toons; party1 (the leader of the party) will not always be one of your characters, and in many cases none of your toons will be the party1 character.

A party based assist (or in your case party based heals) is only going to be consistently useful in 5-man content.
Going with a toon specific target method, 1=ToonA / 2=ToonB / 3=ToonC / 4=ToonD / 5=ToonE, that will always work however the groups are made.
Similarly, with your FTL having everyone assist the leader and heal whoever the leader is targeting... that will also always work.



In general, for heals you can use one of these three systems:
a) Character specific heals, with a given keybind that triggers a macro to heal a specific character... at least one bind per character, but depending on the number of heals you're planning on using it could be three binds per character, meaning 15 macros each with its own hotkey.
b) Assist the mob... by casting at its target or its target's target... /cast [@targettarget,help][@targettargettarget,help] Heal Spell.
c) Using an alphabetical overlay of your team/raid... so the top, bottom or 3rd frame down (on any toon) refers to the same target... combined with a Repeater Region to broadcast mouse clicks to the desired healing targets.