am nervous just thinking about what you said that time machine does not include drives in the backup that are tm drives... why would the genius bar tell me to do that?
The Time Machine System Preferences have an "Exclude" panel in the "Options" panel. It can include external drives in the backup, but initially it will leave out the external drives, and it will not backup a Time Machine backup. Perhaps the genius you talked to misunderstood you.
Yes, let the Genius Bar guide you to restore the iPhoto Library from the old drive. The important thing is to get a copy of the library on your system drive. Once you have the library on your system drive, Photos open the library in Photos, don't try to import it.
Even if Photos should not be able to migrate the library as a library, you should be able to extract all photos from the old backup.