Black screen and drive problems with NTSC-J Model 2 Saturn

The Temperatures of the Beast - after 25 minutes powered on

Dual SH2 (Sega 315-6018): 48°C
VDP (Sega 315-5883): 45°C

The other chips are in the range of 30-35°C.

Is this already a death signal of the SH2/VDP chips? Would be great to get reference measurements from a reference VA15 board...
I don't know what the normal temperature for those chips is, but it's not enough to cook them. The normal operating range of CMOS chips is about 0-70C. I would expect the dual-SH2 to run hotter than the other chips, since it's two dies in one package. Compare the temperatures against your working Saturn.

If you get access to an oscilloscope, check the outputs of the PLL (IC20).