OP - I believe during the 2000 works, the old Carriage line between Park St Jn and Camden Jn was abolished (the tunnel bore at the Euston end is still obviously extent) and the diveunder that is now line X was reduced from double to single track.
Correct, the 'Up Empty Carriage Line' aka the mousehole. It hadn't been used for a while, and was renowned for being a pretty horrible place to be. The track had a tendency to creep down the gradient, and the Pway boys were forever having to pull it back up by brute force.
Line X is located where the Down Departure Line and Up Engine Line were. The original plan was to put Line X on an all new alignment which would have required the intersection bridge to have been completely rebuilt. However it was realised this would only give a very small journey time improvement for services on line X, and would be very very disruptive to construct. So it wasn't done.
Incidentally, the longest all line possession (ie no trains) for the remodelling of Euston was 11 hours on a couple of Sunday mornings. Most of it was done through taking one third of the station out of use for up to two months at a time, with the alterations being part of the permanent timetable for the summer period May - October 2000.
Euston remodelling will have a whole chapter devoted to it in 'A Cunning Plan - the memoirs of Bald Rick'