I would agree, definitely do the circuit. Don't miss out Ghadames though - you could go from the border to Ghadames (via Nalut), then do the Derj-Idri piste from which you can go to Lake Gabroun and the Wadi (if you have time) and on to the Akakus and Algeria.
I am not sure of the impossibility of doing a return from Ghat to the Tassili. We were in Ghat in February this year and Awiss Tours were very keen that we do this option and assured us there was no problem. In the end we didn't do it because we were going to Djanet later. While we were in Djanet our guide for the Tassili from Hotel Zeriba told us that people were coming across from Libya to do this, although we didn't meet anyone who had done it.
I don't know what the Ghat-Djanet piste is like, but we have not heard anything to suggest it's very difficult. I am pretty sure you are compelled to have a guide for this crossing given the proximity to the Akakus and Tassili.
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