Actually is is clear. Congress has the authority to issue subpoenas and the executive has the authority to compel attendence. There is, so far as I know, no limit on whom Congress can subpeona because there is no authority to prevent their consideration of legislation of any kind they desire.
It is also painfully clear that neither Congress nor the Administration had the backbone to follow through because of political considerations.
Please quit blaming the courts for not reaching beyond the pleading filed by Gibbs.
Oh come on. Courts often go beyond the presented arguments for the reasoning of their decisions. I wouldn't call it routine, but it is certainly not uncommon. If ever there was a case calling for it, this was it. The courts had been directed to perform de novo review of this case WRT Terri's constitutional rights. This court showed no deference to the intent of the law whatsoever. We live in a judicial tyranny - the consent of the governed is dead.