How ironic is it that the Haskell Research Compiler is written in Standard ML, and not Haskell? Joking aside, SML is a great language, and often overlooked, so this is good to see!
Haskell was the second frontend. Don't think we ever did a SML frontend. We can't really discuss what the original frontend was so it was scrubbed from the release.
Is that because it became an internal product with competitive advantage or something more boring? I've abstracted away from detail enough so you could hopefully answer.
It's not an internal product but I wouldn't call it boring either. There were some new ideas and some non-technical reasons why they couldn't be used at that time but they could still be viable and potentially something we wouldn't want public.
Not quite; Hotspot (Sun/Oracle JVM) and Android Runtime are both C++; the major C/C++ implementations (GCC, LLVM/Clang, MSVC, Intel C++ Compiler) are all C++ too, although GCC was C until 2010 and MSVC's 16-bit ancestors were as well.