Undecided questions
If "ciao" is for "hello" and "good bye" in italian, how do I say bye?

Arrivederci

you can use for "hello" and "bye" the same word :"ciao"
or you can say "goodbye", which in italian is "arrivederci"

That case never happened to me, as when two persons are
departing from each other, they know!
Bye is an abbreviated form of 'good bye'; /ciao/ is so short that it does not
have an abbreviated form!
If still in doubt, come here in Italy and try it yourself.
Ciao!

When you come:
Ciao (= hello, informal, friendly) , Salve (= hello, less informal and formal)

When you leave:
Ciao (= Bye, informal, friendly), Arrivederci (= See you again, less informal and formal), Salve (= Goodbye, less informal and formal). Ci vediamo ( =See you, informal and formal). A presto, ci vediamo presto ( See you soon, informal and formal).