Regarding the first question, soloswolf's answer seems to be the most satisfying.
The union stack has just entered Winchester from an adjacent region and Johnson's corps was already there in defense mode.
GShock, I must disagree with you on
all-support or all-cav units can enter regions without switching to offensive posture
because it's not what I observe. I'm raiding enemy territories with all cavalry stacks and I can assure you they automatically switch to offensive when region is less than 50% CSA controlled.
Regarding the second question, harbor cities can have depots indeed.
I tried to put the ships in the port (closer than that from the city I can't do) and it still doesn't work. Actually the fleet is composed of three 100% health transports and the region is 100% CSA controlled.
Maybe there must be a land unit in the city in order to build the depot? Or maybe there's a bug with this city.