ASM is really portable since it basically works really closely to the machine codes ( just a step up, in fact ). But ASM would violate Q2.c of your first post, so never mind. Assembly should probably only be undertaken by those who either: a) need it in their job ( I've seen one or two jobs advertised that require the ability to read it ), b) want to have a little better understanding of the underlying hardware, or c) you're just messed up enough in the brain that you'd wanna learn it just because ( like me!
)
But yeah, Java is really portable since it doesn't really compile all the way to machine code ( it is partially interpreted ), and I don't know enough about C# to judge it, but it looks very interesting and I've heard good things about it.