Luebzer,Can I assume that all scheduled stations own a platform g15h, where an entering of SHINKANSEN KODAMA 643 is possible? Coz Ifm insecure from which Mishima Station- platform the Shinkansen does depart.
Platform 15 is the platform number from which that train departs at Tokyo station.
There is no platform 15 at Mishima station. If you are boarding that train at Mishima, you should look for the platform for Tokaido Shinkansen trains heading for Nagiya and Shin-Osaka. If you know the train departs at 10:26, then it will be clearly marked in English as Kodama 643 for Nagoya, so there is no need to worry.
Exists any possibilities to reserve a Shinkansen-seat via phone or Internet? Or is in Kawaguchiko a JR-subsidiary / satellite station located, where I can make reservations?
No, you cannot reserve seats in advance by phone or internet, but since you are taking the Kodama train, which does not have many reserved seat cars, you don't really need to reserve seats in advance anyway. Most people use the non-reserved cars when catching the Kodama trains.
Domo arigato
by Luebzer