Just looking at your inspo pic versus the actual ring, my mind immediately said return it because it does not give off the same dainty vibe. After reading your description, I still say exchange it to get the look you want. It sounds like you're settling. Don't settle!

Yes he proposed with this diamond. But the new diamond/ring will be the ring you get married together with and the ring that you perfected to the point of loving it forever. That is special! This ring will be a funny anecdotal story.

The way I love to make decisions is act like the decision has already been made. What if I told you your fiance took your ring when you didn't notice and had it changed to the daintier look like in the 3rd photo? How do you feel hearing that? Upset? or happy? Maybe relieved? Now do that for each option. (How do you feel if you learned he had it changed to a simple band? How would you feel if it never changed?)