Energy Tornado thing is not an ordinary Tornado but something similar to that column of Energy flowing out of the Nexus. That isn’t going away either.
Cracks spewing out Energy continue to spew Energy and that includes the Cracks inside the Twilight Highlands.
Malfurion for what it’s worth got rid of the actual Tornado in Darkshore.
Windstorms can be dealt with. Cracks spewing out Energy will continue to spew out Energy.
Places Corrupted by Dark Magic will continue to be corrupted unless someone actively purges the Corruption which apparently the Mantid have not been doing(they spend their time stuffing N’Zoth’s corruption into the Kypari Trees over purging the Sha of Fear’s taint).
As for the Nightmare’s corruption of Val’Sharah: Malfurion is stated to be unfamiliar with the Emerald Nightmare’s Corruption not that it matters to him since N’Zoth is dead freeing those corrupted by the Nightmare of the Void’s control while retaining their mutations.
Malfurion did not think Felwood would ever heal so it’s no surprise that Fel Corruption is never leaving(especially since Tyrande’s suggestions of contacting Alexstrasza or Ysera were interrupted by Maiev Shadowsong’s agent).
The only part of Felwood purified Whisperwind Grove was only purified under the guidance of Xaravan who was a Satyr who did it to dupe the Druids he had working for him. It’s easy for a Demon to remove Fel from an Area considering the Demon could simply draw it into himself!
Demon Hunters can’t handle the amount of Fel Consumption necessary to purge the Fel from Faronaar, Felsoul Hold and Broken Shore. Furthermore their base of operations is a Fel Volcano so the only place they might even want purged of Fel Corruption is Felsoul Hold which is on their Nightborne allies’ doorstep.