Ignoring a witch who makes multiple escape attempts, can make herself and others unnoticeable and is invisible to Alice does not strike me as an argument against the presence of an idiot ball.

They would be idiots if they willfully ignored her. They aren't. Her power is to be ignored. Occasions when she chooses not to be ignored are rare, and they can't get rid of her without ruining the effectiveness of an important witch of theirs. (Do they wish they'd kept James, now, so they could kill Allirea and Demetri and replace the latter with a less effective tracker with a more convenient (dead) mate? Yes, yes they do, but they didn't.)

hundreds of years ago when they only had one or two uncooperative witches

Adelaide did not work for the Volturi hundreds of years ago, and so they had no reason to keep witches in the basement in bits.