Hey Berries!
Inappropriate to use Arwyn on a girl - yay or nay??
On one hand, I figure that if girls can be named James than this is fine.
On the other hand, would it be a culturally / linguistically insensitive choice given that Y denotes masculine here (even though in English it often denotes feminine).
I wouldn’t but it’s not the end of the world.
I don’t see an issue, but I wouldn’t do it based on what you said, that in this case the Y denotes masculine.
I am not Welsh, so take my opinion with a grain of salt! This is based on my lived experience in the US.
[name_f]My[/name_f] name is [name_f]Brynn[/name_f], which is a Welsh name with the masculine Y that has become acceptable as a girl’s name outside of [name_m]Wales[/name_m].
When I have talked to people who are Welsh, at the most they’ll chuckle and say “that’s a boy’s name”. I haven’t encountered anyone who has been offended by this.
And as someone living in the US, the large large majority of people have zero clue [name_f]Bryn[/name_f] or [name_m]Arwyn[/name_m] are boy names to begin with.
@Kipperbo1 [name_f]Love[/name_f] this “real world experience” feedback - thanks for sharing! I’d never thought about the origin of [name_f]Brynn[/name_f] (even as a name nerd).
I think outside of [name_m]Wales[/name_m], [name_m]Arwyn[/name_m] would likely be interpreted as a variation of [name_f]Arwen[/name_f] (as in, Tolkein’s Arwen) rather than the masculine name. In a similar way as [name_m]Emlyn[/name_m] might be regarded as a variant of [name_f]Emmeline[/name_f] over a traditional Welsh boy name 
I think it would probably be fine
I’m torn like you - I like the look of [name_f]Gwendolyn[/name_f] etc. more than [name_f]Gwendolen[/name_f] and at one point I liked [name_f]Bronwyn[/name_f] better than [name_f]Bronwen[/name_f] too. But in the back of my head I feel like I hear my Welsh ancestors screaming at me from their graves 
I feel like I’d probably do [name_f]Gwendolyn[/name_f] but other than that I usually stick to the -en endings. [name_f]Ceridwen[/name_f], [name_f]Arwen[/name_f], [name_f]Anwen[/name_f], [name_f]Arianwen[/name_f], [name_f]Briallen[/name_f], [name_f]Bronwen[/name_f], [name_f]Branwen[/name_f], etc. I also think [name_f]Arwen[/name_f] is more familiar via [name_m]Lord[/name_m] of the Rings… so you might see some misspellings that could be frustrating. I think it’s probably fine; as a rule I try to avoid it though. lol.
Good luck!