Huh, I did not realize until reading this that the IME is also x86. I assumed it was just whatever was most convenient, which seems like it rules out x86, but I guess not.
It was initially ARC, but transitioned to x86 with version 11.
IIRC some of the variants used embedded SPARC, though they are a rare find.
AMD uses ARM for that purpose IIRC.
No, the role of the AMD's Platform Security Processor in the boot process is completely different from Intel's IME, as PSP is located on the CPU side and so entire boot process and security checks are completely different and would require a separate writeup.