You wrote, "The withdrawal was proceeding well until the bridge blew prematurely, however. It was the bridge destruction and the trapping of the French rear guard elements that turned Leipzig into a definite defeat instead of an excellent defensive action and withdrawal."
This is exactly the view expressed in the Oct. 20 bulletin of the GA (which is N., of course.) And, it tells us that everything was going according to N.'s plan of retreat up till the premature blowing up of the bridge. This also tells us that it was N.'s decision not to have temporary bridges built by what little was left of his engineering corp; instead, he had them mine the bridge.
So, Berthier is off the hook. N. got the retreat he planned for... except for the Kaboom at the wrong time, of course.
It was a highstakes game that N. was playing. If his great Leipzig escape plan had worked, he's again hailed as the greatest genius of all time. With the premature explosion, N.'s gamble turned into a spectacular failure.