...but depicting Saavik so strongly as an adopted daughter of Sarek and Amanda just makes the whole Saavik/Spock dynamic incredibly incestuous.
By our culture's standards, yes. But terms like "daughter" and "wife" are merely English translations here; the Vulcan concepts and definitions of family may be rather different, with the English terms being just rough approximations. After all, your son's wife is considered your daughter-in-law. If it's okay to consider a non-blood relative a sort of daughter after she marries your son, then logically it shouldn't be any different to consider her that before she marries your son. That logic seems strange to a human, but we shouldn't assume that other species are obligated to conform to our values and assumptions.