With regard to the ships: ARC shipping seems to have a contract with the US Ministry of Defence and their ships visit Antwerp and Bremerhaven almost weekly. Also with regular commercial goods.
ARC Endurance was not in the schedule this time, probably because it was exclusively chartered for this mission. Maybe they wanted to keep its passage secret, although the social media accounts and websites of various US Army entities spilled too many beans for that
With regard to Antwerp, I do not think they have the layout or capacity to handle the off loading and flying out of these quantities of choppers?
Erwin