I would also pronounce
Ewan and
Eoghan differently. I don't know about North
America but in the UK
Eoghan and
Eoin is pronounced the same as
Owen.
Sian is definitely a girl's name. To use it on a boy would be the equivalent of naming a boy
Jane. If you like the sound,
Sion is the Welsh equivalent of
Sean and pronounced the same (in England, anyway).
Shan might work for a boy if you pronounce it Shann, (whereas
Sian is Shahn).
Of your list I like:
NiamhCarys and
CerysSian
Lachlan - I don't like
Lochlann as it looks made up to me and I think
Lachlan is a nice name and quite unusual (except in Australia)
Calum (also like
Callum. This spelling seems to be more popular at the moment)
OwenRossCameron