My thoughts as a former Morgana main(it was quite a while ago, but she really hasn't changed significantly in terms of her abilities)
I'm not sure I see the problem with her passive. If a champion has high innate sustain, then they will be difficult to wear down with damage over the course of a lane; but is that a reason to not design champions with high innate sustain? The fact that she can only do significant damage by either using her bind or walking up and ulting means that most mage champions can simply make favorable trades taking out say 20% or so of her health repeatedly essentially for free as long as they dodge the bind; if she takes the initiative to use a bind and misses, she also opens up an opportunity to be punished. As a result, she typically cannot do much except farm until she feels the need to all-in or hit a lucky bind. While i dislike the centralization of her gameplay around landing a bind, I feel it's fine where it is now.
In my opinion the truly large flaw in her design as a whole is her ultimate; it really needs to trigger the second portion on at least a person or two for her to be effective late game, but it really just lends itself to people saving their flashes or built in dashes/blinks and basically invalidating her ultimate as just a single use nuke. This problem is also a huge contributor to her reliance on her Dark Binding later in the game to function at an acceptable efficiency. The most recent tweak to her that gave her Tormented Soil % damage is a good step to reducing her need to consistently hit bindings, which is good for making her it more balanced.
I've personally found building AP very rewarding on her, though I do admit if you runt he numbers it's probably less efficient than many other mage champions. But I think that's really not an easy issue to address.
For your passive proposition, I really like the flavor aspects you considered in making it. Considering my personal experiences with Morgana, I think while it would be nice for lower levels to allow her to fight and have more dynamic gameplay, it murders her laning phase at higher levels of play, where against a good 90% of players or so you will never land a bind on anyone who is not either under crowd control already or cannot see you and is reasonably close to you. As a result her laning, not considering the presence of junglers, is entirely dependent on farming with Tormented Soil and reaping the benefits of spell vamp simultaneously. Forcing her to trade with enemy champions adds damage to her all-in, but turns her into an all or nothing champion as she will not be able to trade damage favorably against most champions.
tired have to get ready for thanksgiving things more thoughts later maybe kbye