Pretty easy to test if it clicks everything at once, or one thing after another.

Make a macro, which reads:

/Castsequence Curse of Agony
/Castsequence Corruption
/Castsequence Immolate
/Castsequence Haunt
/Castsequence Unstable Affliction
/Castsequence Shadow Bolt

Don't include any comma's in the sequences.
If all buttons are clicked at once, then the later spells should not fire.
I could see server latency possibly letting a Corruption go off, but probably not anything later in the macro.



Personally, it makes sense that it tries to do everything at once.
On click #1, the first sequence fires off.
However the 2nd and subsequent macro's fail, because the first has incurred a GCD.
We know a castsequence will not skip a spell, so these later sequences are still on the first cast.

On click #2, the first sequence is a null, which allows the second sequence to act.
Etc.

I could easily be wrong, but that makes logical sense, and seems to be true to my observations.