If you want to nerf Morgana in the support role, you are doing it wrong.
TL;DR I want to be able to play Morgana in the mid lane, as she was originally intended. I don't really care for support Morgana and agree that she might be a bit too strong.
Morgana doesn't use Tormented Soil for damage in the support role -- she uses it to tag people for assists and to spend Spellthief's Edge stacks.
By removing early damage from Tormented Soil, Morgana will be pushed in more easily in the mid lane. Her wave clear was already abysmal, taking many seconds to work whereas other champions can instantly clear a wave. If you are against a champion that instantly cleared waves, then chances are the enemy minions would walk off of your soil forcing you to tank the minion wave and spend another 130 mana dropping another patch of soil.
The best way to nerf Morgana's supportive capabilities is to remove the interaction between Spellthief's Edge and Tormented Soil. Make it so that it doesn't spend all three stacks in one cast. That way, Morgana spends more mana constantly dropping soil OR has to blow her binding, providng a significant moment of weakness for the next 11 seconds.
An alternative to the above can be buffing Black Shield's cooldown to 15 seconds, giving it an offensive use (aka, use it on an enemy to spend the cooldown and do some damage/effect.) Have the cooldown doubled to 30 seconds if you use it to shield an ally. The shield duration can also be shorted to 3 or 4 seconds, if needed.