Which temperatures can be considered as "safe"? / Which temperature thresholds should I use?

I don't know. No such information are published by IBM/Lenovo.

Try running "stress -c <number of CPUs>" (this program just uses the CPU and so generates heat) when tpfand is disabled (i.e. "Control fan by software" is NOT checked) for about 1-2 hours and see how hot your notebook gets. It's a good guess that these temperatures are safe, because they can be achieved during usage of the notebook under normal circumstances (i.e. fan controlled by embedded controller).