Is it productive to society to convict someone of, say, "hiding one's identity in furtherance of a computer crime" (or the actual legal equivalent of those made up words I just typed) if that "computer crime" wasn't proven?
Not really, but there are statutes - such as "unauthorized access" which become felonies if attached to any (even minor) other crime. So you can take two minor crimes, join them and get a felony. Moreover, if both are of such nature, you can have two felonies now, and if jury returns guilty on any of them, I understand you'd have felony conviction.
Presumably if that particular combination were in question, there would be no way to prove the former without the latter. That doesn't hold for all the combinations the prosecution filed.
I don't think this is true. I think they just want to make sure they can convict him if they can prove lesser charges but can't prove the main charge.