1) Mojave DP3 has 0085 version to update any previous BootROM.
View attachment 769058
2) Didn't notice, but I'm not using betas to work. Betas are problematic to benchmark, so much logging and not yet optimized code. Maybe someone did some benchmarking and can answer this question with numbers.
3) Yes, the BootROM firmware initializes the CPU, the OS can update it later - macOS and Linux do the microcode update. Windows seems the problem, various people reported that Windows didn't boot after 0087 was installed, so we may have a problem here.