If you go with a single macro which reads:

#show
/cast Flame Shock
/cast Lava Burst
/cast Chain Lightning
/cast Lighting Bolt

The Blizzard macro system executes top down, but tries to do everything at the same time.

If it were sequential, then four presses of this macro would get four spells, one after the other.
Because they're all pressed at once, but the execution is top down, only the first one casts.
The second, third and fourth will fail, because the GCD is already used.







If you go with a castsequence which reads:

#show
/castsequence Heroism, Heroism, Flame Shock, Lava Burst

The sequence will do the first Heroism.
A castsequence will not move on to the next item, until it is successful.
So we'll have to wait for the Exhaustion before the 2nd Heroism/Bloodlust fires.
And after that point, the Flame Shocks and Lava Bursts can cast.






Click doesn't change the mechanics of /cast or /castsequence.

/Click Button1
/Click Button2 (etc)

Would not help us if the macro's were set up like this:

1:
/cast Flame Shock

2:
/cast Lava Burst

The first spell fires off, and the 2nd is stuck despite it being /Click.





Similarly, castsequences without nulls won't work either.

1:
/castsequence Flame Shock

2:
/castsequence Lava Burst

The first sequence fires off each time, and the 2nd is stuck, despite it being /Click.







For multiple sequences to work, the first needs to include a null or comma.
So it isn't doing anything when it is clicked.

By doing nothing when clicked, it allows the second sequence which is clicked at the same time to do something.
When neither the first or second sequence are doing anything when clicked, this allows the third sequence to execute.

The /Click mechanic presses each button at the same time.
Blizzard UI attempts to execute these simultaneous presses on/in a first come (top down) order.

The second, third and fourth sequences do not progress from Lava Burst, Chain Lightning or Lightning Bolt, because these spells have not successfully fired off. As with other sequences, until the spell executes, the sequence will not advance.





That's my understanding of how Click Castsequences work, along with the Blizzard macro system.