Responding now because I know that people have a tendency to not edit things later even when they mean to >>
I think you missed a bit in the post- I do mention some applications of using the shield on an ally instead. However it should be noted that balance is an entirely separate consideration from this; we're dealing with the underlying structure behind the kit and what interactions it offers. Numbers can and will be tweaked separately.
Additionally, as I say in the post, she does still have some sustain from the passive; it's just based on the opposing champion now. However, Bullet answers your other questions.
More importantly:
The difficulty or innate-ness isn't the problem; the problem is that it discourages interaction. Especially in the early game when most mid-laners will be severely limited in how many spells they can cast by their manapool, throwing damage at a target that's just going to heal it back effectively passively contributes nothing to the actual 'fun' of the game.
As a side note, Nasus's passive is less problematic because he has to at least be up to the minion wave and actively AAing to use it- by contrast a low-health Morgana can cast W from 900 range and then hang back and soak up the regen from complete and total safety for the next five seconds (granted she's missing last-hits, so the opponent does still gain something from this but the interaction is not as valuable as it could be.)
I agree that the ult does currently contribute to over-centralizing her to landing Q's however I think that the ability is still a good ult- possibly Morgana's best-designed ability, in that it has a very clear effect with a wide range of counterplay on the opponent's part (and trade-offs: if they save their dashes, etc for that then they don't have them for other purposes) and both Morgana and her opponents always aware of it and ready to respond to each other.
Further, my proposed changes do help to reduce reliability on Q because they add another source of damage- the mark triggered by the slow; even if they break the tether range there's still the additional chance for damage from the Mark of the Fallen if she can toss a W down onto them first/after
So, I did make kind of a mistake here and was basing my assertion on her old W's effect. I just ran the numbers for the new one and her single-rotation damage is now actually quite high (although as it is largely damage-over-time, this is appropriate). I will restructure my argument accordingly.
However, I disagree with the argument the implications of the final quoted paragraph. If we're assuming she's in a carry position- and she should be for thematic reasons I've already discussed- then she needs to be able to do substantial AP damage. "Burst" was a very poor word choice on my part, however. The fact that her ultimate is presently mainly useful for scrambling the opponent team (its utility) is what is relegating her more commonly to a support role currently. If it's going to apply the proposed passive's debuff, then that adds further value to it in her role as a carry.
This is extremely feedback valuable to me; thank you.
So, in conjunction with my above concession regarding her damage, then I have to revise my assessment of her second core issue:
Her Q is unreliable, making her damage output the same.
Changing the passive decentralizes the Q for damage in team fights but does nothing for her laning against high-level opponents. Ultimately, the problem remains that she has no practical offense in-lane. Even with my shield/passive proposition that still relies on the enemy aggression (and thereby contributes further to the anti-aggression problem she has in the first place.)
Taken from the patch notes of her previous changes; Riot says they would like her to be more aggressive. Thus, the Q is going to have to be changed too.
I have thoughts based on what I've been comparing all day, which I'll post separately- but I've had this window open for about eight hours while I, too, got distracted, so I want to uh, not lose my comments.