To my knowledge, the surname is also not ever pronounced in a way where the second 'e' would make a sound denoted by 'i'. Rather, it's the unstressed first 'e' that can mutate into the 'ee' sound.

For transliteration sure.. but both of the letters in question are pronounced differently.. The first е is unstressed but the second е is stressed. This would make the first sound more like an i, especially for Russians, which would make more sense if they had written Zilensky based on how it sounds when spoken

Either way, call it a little typo

Because English doesn't have any letters that are pronounced in different ways depending on the context. Neither does Cyrillic. That's a unique thing to the French.