If they refuse to do a full revert, i think the best course of action would be to simply revert his voidlings to a similar state. The current cast voidlings being a core portion of your kit is not a design to balance around.
The real issue with his passive is that it is either going to give him enough time to amass his voidlings, or it will not.
Reverting them to a secondary role, unless building AD, and a passive based on # of spells cast is probably the only way that riot will ever find a way to balance the new kit (oh and add back attack targeting). Then what do you do with his W? Give it the fiora treatment, put his current passive on his W. He's intended to be a kind of bruiser/mage now (as opposed to his old identity... r.i.p.), so give him an actual defensively focused ability for the malzahar player to maximize.
Now doing this would obviously mean you'd probably need to buff his E, Q, and/or R damage to adjust for the lost voidling damage. However, this could actually successfully restore his old farming pattern.
From a power prospective this change would put a larger divide in champion skill of bad malzahar players and good malzahar players. A good player will block correctly and see impressive outplay potential, where a bad player will misuse their W and basically have no defense.