"a major character will be leaving the story"
Definitely a concern, but I'm going to go with the "We can keep him, for now" slant and assume that Amanda's interest in MC creates stress for Mallory and Amanda's relationship. The result is Amanda leaves to focus on experiencing her new opportunity post-Rett.
Heavily invested in a Mallory arc ... She's so damn quirky and likeable. A Mallory exit even for much of a chapter would be difficult.