I'll elaborate a bit, let's use the Sigma Bastad (launcher) its All class & can use Divine Launcher regardless of class. The "bonus class equip permissions" refers to every class that can't equip Launcher, so if Extended it will only be equippable by Ranger.
The "innate PA" here refers to Divine Launcher, so you won't be able to use that PA with it unless you're Main or Sub Ranger.
While Extending there is a chance during Great Success to add a class or 2 back & sometimes all.
Lastly the highest power of an extended weapon is about slightly under Ophistia's +35 Power.
I'll note though that the (Sword) Occuld Diene+35 - 2173 | Extended Sword+35 - 2215 | Ophistia Calibur+35 - 2303
Additionally regarding the Millioraj Dusk (All-class Harmonizer) it has dropped in Disturbing Whistle of Chaos on UH, so not solely Rainbow Keys.