This is a great topic and what you've put forward is a serious meal for thought.

For me it's pretty clear. Seduction is not coercion, it's play. If seduction involves true manipulation, it is not seduction, or at least not only that. Thus defined seduction is always ethical.

The reason for this is that the seducee (if that's even a term) has already been moved by their own impulses, desires, etc. which originate within them but in response to the other. The seducee derives extra pleasure in being seduced, as does the seducer in their role. As you mentioned, like hypnosis, it can only occur with willing participants.

The dark side of this is that the play is actually modelled after real coercion. But this is not unusual in sexual activity and play. Certainly one school of thought sees sex as the safe space which dangerous things can be acted out. Of course in many ways sex is anything but safe (think of pregnancy, STIs, strong emotions, etc.) but in another it is ideally practiced within an environment of trust, or at least of self assured power.

So this is where the play aspect comes in and it may not be clear out of context if it's coercion or seduction. This is why we can have a reasonable doubt. But that's play for you, often ambiguous, dangerous and reckless.