I think you articulate this all very well, and it puts me in mind of certain cases in which I totally see why it is Willoughby who is despicable, and Eliza the victim, especially as Willoughby is a repeat offender.

When anyone (and it is worse for young people who have never had the experience) is in a position where ambiance, the moment, the situation, is such that it seems perfect and right and the only thing that matters is making this other person happy, regardless if legality isn't quite right or the formality isn't followed (especially if the other person who you admire and look up to or whose opinion is necessary to the future prolonging of this happiness doesn't seem to think it that important), - it would be hard to say no in such a situation. But those situations happen in two ways - 1) it is either non-premeditated and "meant to be" (ordained by a high power - perfect for that reason) and it will ALL work out for the best, OR 2) it is premeditated and in order to get to that "perfect moment" some observation and practice is needed to get to something that seems perfect - particularly as a show to another person who has never felt that way before. 2) is despicable. We know with Willoughby is was scenerio 2, because it happened twice, and HE broke it off both times (or three times, if you count Marianne.)

It takes discernment by outside checks or wiser minds (such as Elinor or Col. Brandon or a father) who would be able to suspect all is not right, and they can act more as the "bad cop" by curbing the exuburance until the legalities or formalities have been met. I am reminded of a tangential idea just occurred at work with me - I project manage electrical jobs worth 100s of 1000s of dollars. We submit engineering drawings ahead of time, but if the customer really wants the gear quickly (like before the end of the year), we try to streamline the process by reviewing drawings and bills of material (BOM) together, notes are made but formalized later, but a formal approval is made following up with corrected official documentation. There is a consultant involved, and the customer. If the customer does not approve the drawings and BOM, we are not authorized to order material. BUT we were burned once in a previous job due to the fact that the consultant approved the drawings, and we proceeded. But there was a hiccup, and changes were requested (but we had everything on order to build). Due to the fact that the customer did not provide written approval, even though they verbally went along with the consultant... it was at our expense that changes were made. I almost made the same mistake this week, since our sales person was eager to move forward, and frankly even the customer wasn't aware that they legally needed to approve (or at least that's how it seems - no matter whether it was intentional or not.) I was naive on it, but I was guided by those who knew better. Whilte I wasn't in a "romantic" moment, there was a sense of urgency. It would be harder if this were a romantic situation with no mentorship! Our customer is not premeditated, but it would have been fishy if they had been unwilling to formally approve, or if there was previous known situations with this customer similar to where we found ourselves with a previous job.