[He personally thinks that Chara would do better than that. No matter how much they front, they are not the manipulative beast they say (and think) they are. But, even if they were, Mettaton cares about them AND Alphys. He wants them both to be happy. Even if he's pushing for one side doesn't mean that Alphys is any less important to him.

...Even though he's been pulling away as of late.]

A letter could do nicely! It spares you both a direct confrontation that might escalate things, and it gives you time to clearly dictate what you mean.