I could go with the straight Lava Burst spell and my problem would be solved but because I use one button for Flame Shock, LvB, and LB spam for raiding (and to an effect pvp). I don't enjoy mashing 3 different buttons that can possibly screw up.

Edit:

1 - Your cast sequence syntax is wrong which is probably breaking it, you don't need the slash.
/castsequence reset=6 lava burst,,,,,,,,,,,,,,,,,,,,

This works fine.

/castsequence [target=targettarget,harm] reset=6/target Lava Burst,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

This does not and I want to know why.