Stephanus is from the Greek meaning crown: στέφανος. The "ph" would not be "f" but instead like the ph in "uP Hill."
That's interesting that they use "Ego" like "Εγώ" in Greek. Both mean "I"
It does suffice, but including the "ego" is not strictly wrong, and so should be accepted regardless.
Latin drops the pronoun, not the verb, so it would be "Sum Stephanus/Stephanus sum" which are both accepted (though unfortunately it takes a while for the changes made in the Incubator to become active for users).
"Ego sum" is emphatic, as you said, so since there's no context it's a valid translation.