Will my initiating spiritual master have to take birth again to rescue me if I don't return to the spiritual world?

Hare Krishna! Thank you for your letter. It is wonderful that you have taken initiation in this life...That will certainly mark the beginning of your journey back to Goloka Vrindavan and it is certainly true that there is some possibility that you will not make it in this one life.

However, the relationship between the guru and the disciple is eternal. Somehow the Guru will continue to give instruction as long as you are sincerely attempting to follow his instructions. If you become negative toward devotional service, or commit sinful acts deliberately, then the guru's obligation will be finished, although out of affection he may come to save you anyway.

Here is an experience with Srila Prabhupada in that regard (from a 1996 Vyasa-puja offering by Nikunjavasini dasi):

"I was coming and going, as my small baby kept making noise. Then a devotee asked a question: 'In the guru-puja song we sing, ‘You are my spiritual master life after life.’ Srila Prabhupada, does this mean that if we don’t make it this lifetime you will come back to save us?' You answered firmly, banging your fist on the little bookstand before you, 'You make it this lifetime, don’t make me come back.' Again we must remember you gave us all the tools. We have to understand how important it is to follow you and not get distracted by the agents of Kali.

"To become your disciple, it is not enough to take initiation from you. We have to deserve it by taking to your discipline. 'Disciple' means 'strict follower of the disciplines given by the spiritual master.' Just as a doctor can help a patient only if the patient follows the prescription and diet the doctor gives, so you can save us only if we strictly follow your instructions. Srila Prabhupada, you gave us everything, and we have a lifetime to practice accepting your gifts. You are an ocean of mercy, and you rain it down on us without restriction. May we develop this faith: that if we follow you, you will take us back to you after our time in this body is over."

I hope this is helpful
Sincerely,
Laxmimoni dasi