"Is she the one who calls you?"
Translation:Is zij diegene die jou belt?
a). No, you cannot. "Wie" is not used as a relative pronoun, only as an interrogative one.
b). "wie je belt" is a subordinate clause, so the verb comes at the end.
"Is zij degene die je belt" can mean either 'Is she the one who you are calling' OR 'Is she the one who calls you'. If you want to avoid the ambiguity, you can replace "je" with either the subject pronoun "jij" or the object pronoun "jou".
Here the DL English sentence says "the one" but the translation into Dutch says "diegene".
You are correct that Dutch "diegene" literally means "that one" rather than "the one". However, a native English speaker is very unlikey to say "that one" in the sentence here. That would not be incorrect grammatically, but, at least in modern English, it would soud quite formal and/or stilted to say: "Is she that one who calls you".
In Dutch, however, there is nothing odd about using "diegene" in the DL sentence here. So -- and this is the point that you seem to be missing -- in the same linguistic situation, when an English speaker would certainly say "the one", a Dutch speaker might very well say "diegene".
Of course DL should accept as correct here "degene" as well as "diegene", since the former is also an accurate translation of the English.