I've not read your replies so please excuse if I repeat...
Allegra - "
Happy" is my first thought, though you may not want that used for school/public; "
Lea" pronounced like
Princess Leia? "Lera", to rhyme with
Sarah?
Calista - "Cat", "
Cass", "
Alis" (
Alice), "
Liz", "
Lisa", "
Lia"
Juliet - "
Jules", "
Etta", "
Ettie", "
Lettie", "
Li" or "
Lee", "J", "Juju"
Sapphira - "Phi", which is what I would use for
Sophia (in my family "so-FIE-uh"); it's a Greek letter but also used in math for various applications, including as a symbolic representation of the golden ratio
Valentina - "
Nina", "
Lena" or "
Lina", "
Vanna", "Tinka" or "Tink"
Seraphina - "
Ina", "
Fifi", "Seph" or "Sephi"
Best I can do. :)