ARM can emulate z workloads. https://hercules-390.github.io/html/index.html

(or QEMU, for partition level emulation).

There's also the poorly advertised IBM-native S390 emulator. I worked on it over a decade ago, you could run S390 on PPC and X86 offically, and ARM and a few others were unofficial.

Sure. And you can run ARM workloads under QEMU on s390x Linux.