I'm mentioning 3 scenarios that may occur during public Torah reading.
1 - The only Cohen in the shul is in the middle of the silent Amidah. Should you wait for him to finish, and if so, how long? Or is any waiting, even 20 seconds considered torach tzibbur? If you don't wait, should you call a Levi or Yisra'el?
2 - The Cohen had his aliyah, the only Levi is in the middle of Amidah. Should the Cohen take the Levi Aliyah or should cong. wait for the Levi?
3 - The only Yisra'el in shul is in the middle of Amidah. AFAIK, a Cohen or Levi cannot take a Yisra'el aliyah. But waitig for the Yisra'el to finish might be torach tzibbur (say he's at the beginning). Do we still have to wait for him, or is there an exception to use a Cohen or Levi to sub for him?