Not a big fan of “quickened” giving you different types of actions depending on the source. Really unintuitive and messy, and basically means that haste and that monk ability don’t actually give you the same condition, but it has the same name anyway. Would quickened just giving you an additional action you can use for whatever really be that overpowered?
Probably not, definitely something to mention during playtest
I dunno. Casting 2 9th level spells in one turn seems pretty potent to me.
Considering that it will require a caster at least level 17 and spells per day is capped at 3, I don't see any issue in that.