I'd be perfectly happy to limit them to sor/wiz spells.
What if we say arcane but allow bard spells as well?
Right, Still Spell should surely be out.
I think I see what you're getting at (Cleon) about the casting time, but it's taken a few re-readings. You want them to be able to voluntarily reduce the spell's CL, like a sorc/wiz can reduce the CL of a scroll (for ex) to decrease the casting time even more. Is there any way to clarify this further?
Indeed, bards cast arcane spells, but Shade wanted to limit to sorc/wiz only!
I didn't find your last revision clear enough, but maybe that's just because I'd come in with some preconceptions about where we were going. I think it's ok.