On the word χριστιανός/χρηστιανός BDF (footnote 3 on section 5.2) seems to indicate that the suffix ιανοί is a Latinism and says that it is "...later much used for names of sects." I'm having difficulty divining the period they call "later." What time period would that be?
Also, BDF on MAJOR VOWEL CHANGES pg 14 (3) section 24 says;
"...there was a phonetic leveling of vowels η, ι and the diphthong ει in the Hellenic era. Sporadic examples of the confusion appear first appears in Attic inscriptions from 150 C. E. on."
It then goes on to say that the spelling χριστιανός in S* (A 11:26; 26:28; 1P 4:16) depends on an interpretation of the name Χριστός based on the similar sounding χρήστος, which was also frequently used as a proper name;"
So is this indicating that though the spelling is different the pronunciation and meaning "Christian" is clear?