Yes, the i9 9900K is compatible with your ASRock Z370 Extreme4 motherboard.
Yes, the i9 9900K is compatible with your ASRock Z370 Extreme4 motherboard.
It would function after the BIOS update. I wouldn't suggest using this board with an i9.
Assuming it isn't overclocked is fine. However, it's safer to be cautious and I'd prefer a board that clearly indicates whether a BIOS update is required.