If the name Yoh.anan (John, Iôannês) were used in Samaritan Aramaic and Samaritan Hebrew, it would be written as in Masoretic Hebrew YOD-VAV-H.ET-NUN-NUN and pronounced YOD-u-‘AYIN (sic!)-a-NUN doubled-a-NUN Yû‘annan, accented on the second syllable. (You will see that I can now make apostrophes go in either direction). Note that the pronunciation of the ‘ayin sound is never weakened in Samaritan Hebrew and Aramaic if the sound occurs at all in a word. (An original H.et or ‘Ayin sound can end BOTH end up as the sound ‘Ayin or the sound of ’Alef or can show up as doubling of the vowel, This doubling of the vowel, that is, a lengthening of the vowel with two tone, level followed by rising, and a primary and secondary stress, is common in the Masoretic pronunciation as well. It can occur in English, but I can’t explain in writing). There would be a diminutive but I can’t say for sure what it would be. Yanna is a possibility. So is Yanni. So is Yûna. This last is the likeliest. In this case, the spelling and pronunciation would be the same as in the name Jonah, Masoretic pronunciation Yôna, Samaritan pronunciation Yûna.
The Rabbinic and Biblical name Yona could be a diminutive of Yoh.anan. I’ve never thought about it. See if Jastrow helps.
Stephan Huller has had a life long interest in religion and spirituality 
dating back to conversations with his grandfather, Gaston Frank. "He said we represent one of the last descendants of the Frankist Jewish faith in the world," he muses. "I grew up thinking that our family was something like the Last of the Mohicans."