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I think that it works!
I agree that if [name_m]Jonathan[/name_m] can go by [name_m]Jack[/name_m], even though [name_m]Jack[/name_m] is supposed to be a nickname for [name_m]John[/name_m], then [name_u]James[/name_u] can too.
Are any of [name_u]Jamie[/name_u], [name_u]Jem[/name_u], [name_m]Jim[/name_m] or [name_m]Jago[/name_m] (all [name_u]James[/name_u] diminutives or variations) possible as a nickname? However in answer to your question I think it’s fine to use [name_m]Jack[/name_m] as a nickname for [name_u]James[/name_u]. If it helps you could think of it as coming through the [name_m]French[/name_m] version of [name_u]James[/name_u], [name_m]Jacques[/name_m], which neatly lends itself to [name_m]Jack[/name_m].
I say, why not? There are plenty of people with nicknames that have nothing to do with their real names anyway. Go for it. If he’s not very old, you could maybe even consider changing it entirely?
Although not intuitive, I think it works.
Another option would be [name_u]Jay[/name_u].
I think it works.
I know a [name_u]James[/name_u] who goes by [name_m]Jack[/name_m]. I think it’s fine and nice to have the formal [name_u]James[/name_u] as an option on paper.
I think [name_u]James[/name_u] nn [name_m]Jack[/name_m] is lovely.
I don’t see any issue with it, I have [name_m]Jasper[/name_m] nn [name_m]Jack[/name_m] on my personal list and that’s always been well recieved so I don’t see a problem. I do think [name_u]Jem[/name_u] and [name_u]Jamie[/name_u] would be a more direct relative to [name_u]James[/name_u] but [name_m]Jack[/name_m] is cool too.