I think the Leah ending should have been a thruple of Alice, MC, and Leah. I always played this game with a harem ending in mind but it never felt like Leah was ever as interested in MC as the rest, so in some ways the ending makes sense in the annoying indecisiveness of Leah. While MC has never been an uber Alpha male character, he ended up seeming really pathetic and out of character in his pursuit of Leah. I don't think the groundwork was laid for his actions to be like this, but again, I played the game with a harem ending in mind, maybe those who played with Leah as the ending feel differently. The Alice-Leah relationship felt equal to the Leah-MC but the MC-Alice was the strongest. Of course MC cares for Leah, but neither he nor Leah seemed invested enough in their relationship for them to be so miserable when they were apart. Since Alice seemed to be an equal in the relationship with Leah, I expected her to be in the ending.
When it comes to the group, I felt Alice was easily a choice for a couple ending with MC, Melody and Karina I could also see with just MC but Leah seemed like she was going to be part of the harem or a thruple, but never a couple. I will try to replay the whole game when its complete to see if that changes how I see Leah and her place in the story but as it stands now, this ending felt out of place.