Another thing to consider in your orchestration is the timbre, more specifically that it changes as the part goes towards the extremes. Do you really want the sound of a mezzo-soprano trying to hit an A5 just so that you can fill out your chord? Be aware of the blend you can realistically achieve with the part you've written. The more range you include in the mezzo part, the more difficult it will be for a human singer to produce the tone you desire.
Additionally, you may want to go find a friend who can sing this and ask for their opinion. Even if you yourself are a mezzo-soprano, it's always better to hear others' opinions. The composer may overestimate the abilities of the performers they write for, as they didn't have to "learn" the part, because they wrote it around their own voice (perhaps subconsciously).