Welcome! I'll try to answer your questions as asked. Good news, most of this is working normally.
1) Normal. Your PSU software voltages appear normal. These are software readings and a multimeter may reveal a closer reading to 12, 5 or 3.3. Either way, this is normal for any PSU and not related to the motherboard at all.
2) Abnormal. This could be caused by either the CPU or motherboard settings. It is possible your CPU is poorly binned and has a high default value. Usually, at default Auto settings or XMP you see 1.24 with a drop to 1.20 under load (LLC). If this is related to the motherboard settings you can clear it by restoring defaults in the UEFI or possible clearing the CMOS by the button. You may even need to clear and remove the battery.
3) They are not old at all. The latest bios is 3504 dated August 2017. ASUS has released 17 different bios versions for this board. At this point any oddities or compatibility issues are ironed out with the vast majority of hardware configs.
4) XMP should be working fine with G.Skill memory in that config. What speed is the memory reporting after XMP is enabled? It should be 1700. The BCLK at 100.4 is normal.
5) The Maximus VIII Extreme and Extreme/Assembly can be compared at the specification pages. Just visit asus.com or google each model name for quick results. The first difference is the look, followed by gamefirst LAN, then the inclusion of the asmedia USB 3.0 controller, hi-fi kit... etc. Compare here:
https://www.asus.com/us/Motherboards/ROG-MAXIMUS-VIII-EXTREME-ASSEMBLY/specifications/https://www.asus.com/us/Motherboards/MAXIMUS-VIII-EXTREME/specifications/I think you are really close to setting it up properly, hang in there. Lots of information here to help!