The first thing to check is vowel lengths.
L&S indicates that all vowels in presbyter and presbyteri are short apart from the genitive ending -i.
In presbyter the second last syllable by is light (also known as short) as it is open and has a short vowel.
Therefore the stress falls on the third last syllable.
In the other forms the second last syllable is te, and it is light for the same reason.
Therefore the stress falls again on the third last syllable, but it is now a different syllable as the endings add a syllable.
Your stresses are thus entirely correct.