Question: Can a SCSI system cable with terminator remain connected to a Macintosh without the other cable end being attached to a SCSI device?
Answer: Yes, the SCSI can be terminated and connected. There shouldn't be any difficulties with this setup.
Question: If a SCSI peripheral is attached (for example, system cable to HD 80SC), can the SCSI extender or SCSI peripheral cable be attached to the last "real" SCSI device with the other end of the cable left unattached?
Answer: SCSI cables can be connected to the last SCSI device in the chain, provided they are terminated.
Question: Does a terminator go on the last device, or at the end of the last piece of cable?
Answer: As long as these cables are connected, they are considered part of the SCSI bus -- whether connected to a device or not. As part of the bus, you need to terminate the the last cable if not connected to another device.