I like both Benjamin, Christopher and Christian. If you like Kit, put Christopher on the BC but always call him and introduce him as Kit... You don't have to tell people 'his full name is Christopher but we call him Kit', what business is it of theirs.
I also love Kit but not Christopher or Christian! In fact, we seem to have pretty similar tastes as I really like all of your choices [unhelpful].
If I didn't have to take DH's opinion into account (who likes nothing), I would say my favourites are Alexander (nn Xander) or Sebastian (nn Seb). DS2 was very nearly Benjamin, but we changed at the last minute as I decided I didn't like the "jamin" bit.