Connecting cell phone and computer audio through TRS and TRRS 3.5 mm audio jacks
They seem to be correctly connected
Mic levels are much lower than line levels. Some audio interfaces have selectable gain control to switch between line level input and mic level input, some don't. Output volume can be decreased to match mic input. It may be a good idea to attenuate the signal for the mic input.
Yes, it is safe to use only one channel and connect directly, except for the attenuation for mic input.
Simply use two resistors to mix audio channels together. Add third resistor for attenuating the output.
You can simply plug a direct cable from line out to line in, that's what it is for. The voltage levels match and you get a good quality two channel stereo link, as mic inputs tend to be single channel mono and more noisy due to lower signal level and higher amplification gain inside the PC.