Maybe the voice actors have been busy with other projects and the creators of the show decided to simply put Diamond Tiara and Silver Spoon in the CMC episodes as voiceless cameos instead of removing them completely.
I think that any character from equestria would become their counterpart in the human world and vice versa. For example, Spike became a dog because he is already one in the human world.
What are you guys talking about? None of the Mane 6 have been replaced. The only difference is that Starlight Glimmer is considered a member of the mane cast now.
For example, did Twilight have a character arc or goal outside of simply wanting to be Celestia's student and learning about friendship? I'm not including Rainbow Dash because everyone already knows that she wanted to become a Wonderbolt.