Clarification on Size of an Encryption Key
Yes, 64 bits is equivalent to 8 bytes, because there are 8 bits per byte.

"Could it be instead that 64 bits has to be interpreted based on an encoding scheme such as unicode that includes many more characters? "

No - the key does not need to be encoded as either ascii or UTF. Typically crypto algorithms operate either on an array of unsigned characters, or larger words when required (i.e. unsigned long long for 64 bit words).