But a simple non-production quality implementation in C++ would also be amenable to experimentation and not have the bootstrapping issues as well as provide an easier starting point to incorporate more of the existing optimizations as desired.
Probably not, because JVM users are much more likely to be more proficient in Scala/Java than in C++.
That is true, but the author of Metascala wanted to write it in Scala. Other people are free to write a simple C++ implementation of the JVM themselves.