About the Death Lv. 5, for the bosses case, instead of making it immune and do nothing (which some people mentioned would take the fun out of it) just make it do damage instead if it fails. This would always fail on bosses but at least do damage, so a BLU base rotation like any other jobs could be possible. The same thing could be applied to other OP skills. These OP skills would work in normal mobs, however, to keep it fun.
As for it being OP in say, dungeons and so that BLU can't spam it over and over, either make it a super long cool down or make it use a lot of MP so that the player decides if they want to sacrifice their rotation for it. Or even better, make it like RDM plays. We can trigger either Verflare or Verholy, of course it depends on our mana bar. In the case of BLU the player could make the decision of which way to go in their rotation. Spells like Death Lv.5 could have penalizations for being used to keep it balanced. This also means that they can't do insane DPS, but more akin to RDM, since they would have support abilities, like idk Mighty Guard.

They could keep mandatory spells out of the 49 for a basic rotation. It does sound like a pain to work around, but I think it *could* work with more back to the drawing board.