I was able to answer my own questions.
1. You do change trains in the Monastiriki train station to get to Syntagma. According to my Greek contact, "you physically change metro line and you get the blue line DIRECTION DOUKISSIS PLAKENTIAS OR AIRPORT and you get off at the next one which is Syntagma. Follow the signs all in English written and you will have no difficulty at all. You get on and off some stairs or escalators in Monastiraki station but it is not a big station."
2. In walking from your ship to the metro station, you walk around the Great Harbor as Tom says. When you are on street Akti Posidonos you come to a big square Plateia Karaiskaki (Tom has a photo). You turn right as Tom says. The street you are now on is Akti Kalimassioti. You walk about four short blocks as Tom stated, and the station is on your right.