i am not sure if J# is officially supported or not. however the case conversion sounds familiar, a few users have reported a similar problem in C#. the problem is that i have never really been able to reproduce this problem myself, making it hard to look into.
how consistent is the case conversion in J#?
when you say that "MessageBox" is not recognised in C#, what exactly is happening? what does the HCB show if you hover the mouse over "MessageBox"?