From what I’ve played, the king never confronts you directly. He just waits until after the Tomas incident and then burns you at the stake if you don’t do anything
Getting into this WAYY later because I didn’t start playing it til recently, honestly I never cared much for de Mendosa- I found him too much of a dreamer, and when I played through Part I, I didn’t see much to gain from him unless I only wanted to pursue a romance/love option (which I definitely did not, all I cared about was having as much power as possible).
I hadn’t even thought that de Vega would even be considered a romance option, but as soon as I realized how useful he was, I made sure that we would be allies throughout the game (he definitely helped me with like 80% of my scheming, which I thought was awesome). Then when the romance part came it was a bit of a shock to me because of the fact that he had a wife, and as everyone had said, the time gap after she dies and he comes to you makes it a little odd if he really did love his wife so much.
However, if you play through it the way I did, where you’re friends/allies throughout the game and have an understanding with each other about working closely together to manipulate the king’s decisions, it actually makes sense that the two of you would in time become attracted to one another, even if it’s not realized until after his wife dies. Of everyone at court, the two of you have the highest influence over the king, and of all the characters, I feel that he’s the most like your equal- assuming you cared for maintaining intelligence and subtlety as much as I did. Because of that, I loved that they put him in as an option, even if it was random at first.
I played through this whole thing as a woman so I wasn’t aware of him constantly being male no matter what, but yeah if it was sort of a last minute change, it makes sense why they wouldn’t want to go through the trouble of going back and changing it in the first two parts. Obviously, it doesn’t particularly bother me either way 
I would say de Mendosa, all the way. I actually thought his eagerness was adorable <3 I thought he would be gone forever, as I had stayed true to the king. But when he finally returned I was like 
As for de Vega, he seems like a much more mature and sane option lol, but it DOES confuse me a bit with how he was so grieved for his wife, and then suddenly is super in love with you. I think it would’ve been helpful if some small hints were dropped along the way in Parts 1 and 2, just to give a little bit of backdrop for the reveal in Part 3.