Swain main here with 130 games on the new Swain so far. As someone who has played the new Swain during all his patches, I strongly believe the only nerf he needs is his Q CD reverted to last patch and here is why.
Swain's early game trading is all about landing his E>W>Passive Pull>Q combo which takes skill and can be outplayed. If Swain lands this combo he SHOULD be rewarded with a high burst of damage dealt but should NOT sometimes be able to then throw out another 2-3 Q's since it's on such a low CD.
Nerfing Swains Q damage and keeping the current CD will mean that whilst his mid-late will be similar, his early laning will suffer since his E>W>Passive Pull>Q combo will pack way less of a punch.
Reverting the Q CD would ensure that whilst Swain can still land his combo and deal considerable damage, he couldn't outright then kill you in some scenarios with follow up q's.
This would create a more fair laning phase for opponent as they can then fight back or escape whilst swains abilities are on CD for the next 7 seconds or so.
With the nerfs coming to abyssal, Swain will soon not be in a great spot if any of his current damage is nerfed.