But if I assume your premise (BTW, I still say the new one doesn't override the old one on this point, but I'm going with you here.) why was this done? Was it sloppy legislating? I doubt it was done on purpose, otherwise Clinton wouldn't have been foolish enough to put the "expiration date" on the letter of appointment.
And this reminds me of all the converts to Federalism we got during the Florida recount fiasco.
If the courts do need to read into it the ussual thing a strict constructionist will do is determine what meaning gives the least power to government. In this case the power to appoint vacancy appointments could be considered to be an extra power and if its not explicitly written into the law then it cannot be assumed. A conservative judge would in essence force congress to clean up its own mess.