I don’t like it with the y because that’s not how it’s spelled. At least I don’t think that it is, except where people make things easier to pronounce by spelling them easier in English.
That said, I did prefer [name]Killian[/name] to [name]Cillian[/name] because most of our latin root words make a spelling/pronunciation pattern where a C followed by an I or E make an ESS sound not a [name]KAY[/name] sound, and no matter what they do in Irish, I think a lot of those names are just not that intuitive with a lot of the Latin root English that most of us speak. English is a tough language to learn because it’s made up of a mixture of a couple other language roots, so there is no actual rule that Ci is [name]Si[/name], or -oe is OH not O-ee, it’s that one pattern is more familiar in daily speech and spelling than another; every “rule” has exceptions, which doesn’t keep people from mispronouncing a name or word. I generally prefer the “right” spelling over the phonetic adaptation, such as [name]Caitlin[/name] before [name]Katelyn[/name], or [name]Michaela[/name] before [name]McKayla[/name], but then I occasionally break my own rule, for names like [name]Killian[/name] rather than [name]Cillian[/name].
I get that [name]Zoe[/name] looks like [name]Joe[/name] and [name]Moe[/name], so your mind thinks [name]Zo[/name] when it’s [name]Zo[/name]-ee, a kind of mental inconsistency between what you know really and what you recognize. That’s just how it is. I think the addition of the y makes it eternally juvenile or diminutive, such as in [name]Joey[/name].
I don’t know if the umlaut is supposed to direct the pronunciation for sure or if it has been adapted pre-computer due to the lack of symbols or systems to create letters which are not part of our alphabet but which are every so often used in our language on a legitimate word or name. At once, I feel like [name]Zoe[/name] is just how the name is spelled and the umlaut would be both pretentious or difficult for things like forms and fields, but also sometimes that the umlaut is really most correct and even [name]Zoe[/name] is kind of simplified for living in the US. It would be hard for me to pick either, and I love the name, but I’d lean towards, without the umlaut e. [name]Just[/name] like some names look made up if they retain their letter y (like [name]Bryony[/name], we discussed recently), the umlaut was charged by many metal bands using it heavily on their names, for effect and not pronunciation, sometimes even on letters that don’t ever have an umlaut, and possibly to be ironic sometimes.
If a name has some sort of diacritical mark or accent, I feel that it is or may be the correct spelling, but nevertheless, complicated by being beyond our English alphabet. If you look up any name and see how it is used in another country in its language, I feel some Americanization is as valid and acceptable, but not things like adding a y to [name]Zoe[/name] to make it look like it’s pronounced, which creates a “lowest common denominator” effect. I think [name]Zoe[/name] is common enough for people to pronounce correctly, and helping people with their phonics in this case (or in many cases) just doesn’t seem to bode well for society.