I know you can go with a two step mapped key.
Where the press of a button or key does one thing.
And the release of the button/key does the opposite.

I've never set Left click as the hotkey, but have used thumb buttons.
In theory it shouldn't make a difference what you use.
But there's a good chance left click will then be tied to this mapped key, and not function for other stuff elsewhere.

I've not used the execute a mapped key, when the mouse is in the area functionality.
You could try setting the mapped key to use Left Click as the trigger.
Two steps, executing a step on press or release.
Have the desired heal on step 2 (release).
Not sure if you'd target or have the team target on Left Click (press) or not.

Lax will probably have the best answer for you.
Or a work around, to achieve what you want.
Often he can make a tweak to the program, which allows something new (requested) to work.