Why is západ in the locative case here but in the accusative case in "Německo je na západ"?
"na západ" is a direction. Often "na západ od něčeho "west of something"/"on the west from something".
"Na západě" is a location.
Otherwise it would be like Bohemia is on the left of the Czech republic instead of part of it :)
Some words in Czech are always in the plural, such as dveře and kalhoty. Bohemia is one of them.
For my edification, "Ceska" is in the genitive case?
Yes, "Česka" is genitive of "Česko".