Will Niranjan, the torturer of the Beings, ever perish? Will all the Jivas ever attain permanent Salvation?

Yes, One day Niranjan will perish. However, it will take some time. This is because he has been blessed by Param Purush to rule over the 3 Lokas for a period of 17 Asankhya Chaukri Yugas (10^27 Yugas).
47 Lakh years make 4 Yugas and 4 Yugas make one
Chaukri Yuga.
10 Sankhya Chaukri yugas make one Asankhya Chaukri Yuga. At present, 4 Asankhya Chaukri Yugas (24%) have expired. That is to say, the earth will yet face dissolution millions of times more. It is only then that Niranjan will perish (End Time).