How do you know if a diksa guru is indeed able to guide you back to Goloka? How did so many (presumably very sincere) devotees accept diksa from gurus that turned out not to be bona fide in varying degrees?

We can easily answer these questions if we understand the actual qualification of a bona fide guru.

Prabhupada is a pure devotee who can take us all back to Krishna. My service as a guru is to train my disciples to serve Prabhupada’s mission so that Prabhupada is pleased. If Prabhupada is pleased with my disciples, they will be liberated. Very simple. During Prabhupada’s presence in this world, somehow he was pleased with my service and he trusted me to train and engage his disciples. I continue to train and engage his disciples.

In the Nectar of Instruction, Chapter 6, Rupa Goswami and Srila Prabhupada make a clear distinction between a great Acarya and a bona fide guru. The bona fide guru trains and encourages the disciples to serve the great Acarya. It is all very simple and effective.