If you are going to decrease the linger time then you can decrease the cooldown at the early ranks rather than increase the cooldown at the later ranks, you don't need to double nerf Malzahar for no reason, just one of these is good enough.
Besides the problem to Malzahar's balancing is the fact that you REMOVED THE LEASH RANGE ON HIS VOIDLINGS, stop trying to nerf everything else. The day you finally decide to give his leash range back to his voidlings you then have to undo a bunch of pointless nerfs because the only balance problem when playing against Malzahar is the fact Voidlings keep travelling rather than snapping back to Malzahar's location, and because there is no leash range this is the thing that makes Malzahar's laning so save and his jungle clear so fast cause when Malzahar could have jungled before you nerfed it into the ground was because he so too efficient where he stacks up the voidlings and then starts walking to a camp cause the voidlings did not snap to Malzahar's position.
Short end of everything, you can undo the nerfs to the pool damage, give back more monster damage to Voidlings, give back his 550 average attack range, and re-introduce the stupid voidling leash range you should have done forever ago so jungle malzahar does not super efficient jungle clears and actually has to stay on a camp until it dies and mid malzahar has to stay near the wave allowing his opponent to interact with malzahar better rather than summon a pack of voidlings then leave to the safety of his turret.
1. Revert attack range back to 550 (optional reverting movement back to 340)
2. Revert Voidlings back to having a leash range again, having Malzahar to have to stay near a minion wave or jungle camp, reduces Malzahar's efficiency and opens up proper counterplay for his opponent, not to mention having a leash range to voidlings makes Malzahar more fair and less obnoxious to lane against; the rework fixed Malzahar's game health issues, but you introduced a new one by removing a voidling leash range and this is the main problem to why you are having a hard time finding a balance point for him.
3. Let Voidlings deal more damage to monsters again with introduction to leash range, he can jungle better but his efficiency is nerfed which slows down his clear that way, but if you want to reduce jungle Malzahar's safe jungling then you remove reduce monster altogether and have the voidlings take increased damage from monsters so in the early game Malzahar has to tank for his voidlings making jungle malzahar less safe and actually take damage in the early game (later on with more ap he will have enough damage so it wouldn't matter)
4. Revert pool damage back to 6/8/10% max health per second since leash range Voidlings have better counterplay to them we can give Malzahar some power back
5. Shield linger time reduce to 0.5 rather reduce to 0.25 second, an effect like this needs some kind of linger time, do not increase the late ranks cooldown keep them the same as currently; with a voidling leash range re-introduced it will fix a lot of the balance issues (like all of them)