If you are creating a single mission, you don't need to merge the campaign layer. By default the game will load up the campaign layer and all its traffic as well as your mission. This can make it tricky, when something from the campaign layer interferes with your single mission, but I haven't had that happen yet.
As for the spawning (you're right about the term
) then you are correct that the player has to get within 20 (I think it's km, not miles, not sure though) to have something spawn. If you had for example, a destroyer with an entry date of May 1 1942 and an exit date of June 1 1942 starting at Pearl Harbour with a way point to Midway, then the destroyer would only appear if the player got within 20 km of it at its spawn point in Pearl and from that point on it would start moving to Midway, but only if the player encountered it between May and June 1942, otherwise it wouldn't appear.
There is however a little tick box marked 'Evolve from entry date' on the unit properties. What this does is to tell the game to track in a table (without spawning or creating any units to keep processing down) where ships are. If you have evolve from entry date ticked, then the destroyer will leave Pearl in May 1942 even if the player is 200 km from Pearl Harbour. If the player gets within 20 km of where the game knows the destroyer would be, then it will then spawn the unit and you can then interact with it as if it had actually existed all along. This is how the dynamic campaign works as well, it tracks where all the ships would be, without actually spawning them anywhere. The entry and exit dates make more sense when you think that the campaign layer has to deal with air patrols increasing and bases being in hands of American / Japanese depending on dates.
As for the docked and then moving, I've not ever head cause to do this, but I believe this is how you would do it:
If you want a ship to be docked at a port and depart on May 26th 1942, then you'd add a ship and set it as docked, with an entry date of 1938 and an exit date of May 26th 1942. Then add a second ship of the same type set its entry date as May 27th 1942 and give that unit a waypoint out of the harbour - make sure you have evolve from entry date ticked. That should simulate a ship being in harbour if the player investigates, or just coming out of the harbour if the player goes in when it is scheduled to leave.
I'm at work at the moment, so I can't check this, I normally would fire up the editor and just test it, but unfortunately they don't let me play games at work (I know! How outrageous of them). It's possible what I've said is a complete load of rubbish.
I'll check this tonight to see whether I am giving you duff info or not.