IS Boxer and Warcraft both execute macros top to bottom.

So initially, you press MF + IS + Wrath.
But MF is fractionally ahead of the others.
And because of the GCD, only one can fire off.

On the second press, MF cannot fire off in the 15 second window.
But IS + Wrath are both fired off, with IS slightly ahead.

On the third press, you'll likely still be on Step 2 for MF.
IS will likely be on Step 2 as well.
Which leaves only the Wrath keybind to be sent to Warcraft.



Looking up at an earlier post in this thread, you can also set a Step to not advance for "x" seconds.
Which if you were spamming a key, would likely guarantee it went off, before you advance to the next step.

You'd want to combine that with Warcraft macros, to the effect:
/castsequence reset=3 Moonfire,Null
So even though the Step won't advance from 1 to 2, until 3 seconds have passed.
You won't cast your MF more then once in those 3 seconds.

At least I think it would work that way.
You might have to adjust the Step 2, to 12 seconds (from 15), to account for the 3 seconds in Step 1.