1) Yes, pins 11 and 12. The SPI pins are hard wired so the library doesn’t expect you to assign them.
2) It does have a built-in RTC but not a crystal so you’d need to solder one to the appropriate through holes and mess with the Time code to use the Teensy RTC instead. It also won’t be nearly as accurate as the DS3231.
As for pins, the DS3231 communicates via I2C which, like SPI, is hard wired (SDA & SCL). EDIT: These are pins 18 & 19. That is the primary default I2C address.