I think this is it exactly. But let's be rigorous:

1. "I am trying to get you to get a doctorate by holding back marriage." Still works.
2. No goal (that I can see), therefore nothing.
3. No goal (that I can see), therefore nothing.
4. No goal, therefore nothing (assuming I interpret 5 to mean that 4 is NOT with intent).
5. "I am teasing you to get you to have sex with me." This probably will not work, but partly because it interrupts the flirting rather than because she knows that the flirting is going on. Depends on how severe the teasing is, really.
6. "I am teasing you to get you to have sex with me." This probably will not work either, same reason as 5.
7. Assuming no ulterior motives, "I am trying to get you to become a teacher by this encouragement" Still works.
8. "I am trying to get you to become a lawyer by this encouragement" Okay, here's an error; it's clearly not that I can tell you what I'm doing that's necessary. Or at least, not alone.

So, with the corrections suggested by doing this, the distinction should be:

If the target having full knowledge of what you're doing doesn't affect whether it works, it's influence. If the target having full knowledge of what you're doing does affect whether it works, it's manipulation.

Or, to get at why one is immoral and the other isn't, if there's deception involved it's manipulation. If there isn't it's influence.

I had a somewhat similar thought. I was contemplating an unusual situation where society considers manipulation acceptable and influence unacceptable -- the relationship between attorneys and jurors in the adversarial legal system. Trying to subtly manipulate the jury through selective presentation of evidence or slanted wording of questions = doing your job well. Trying to influence the jury via bribery or intimidation = jury tampering.