No, it’s not possible. UE has a complete build system that is custom tailored to support specific platforms and compilers, so that alone would make it impossible since there is no Embarcadero toolchain in UnrealBuildTool. Though if you REALLY wanted to you could probably make one. But then you’re going to be up against trying to compile a codebase of 10K source files using an unsupported compiler (and Embarcadero’s compiler wasn’t very up to date with language standards last time I used it, though that was some time ago). Even the Intel compiler (which is extremely similar to MSVC in many ways and can often be used as a drop-in replacement) can’t compile the engine without some significant patching. So basically, it can’t be done.
As for your third question, yes, there is Clang 
Clang is supported on both Windows and Linux.