OOOOOOOOOOH, so now things are starting to go in its place! Spoilers from this episode.
Veronica was not engaged to him from the start because he was already engaged with someone else
They used their power and put so much effort on Elena so that she could gain at least the position of royal consort
Then they poisoned the real empress to give her that place
The crown prince never loved Elena/Veronica, it seems sus his reactions to his newborn son, I can't tell if its because he already was aware of the plan and having a son with a fake was never part of it or it's because he felt the betrayed the previous empress who he could have loved.
I have sus has there is no mention of an investigation of the empress death, as at least finding a made up culprit
I really really hope she does not fall in love with him in this lifetime.