You write, ". . . he ignores beforehand the sound advise of some experienced officers?"
Nothing new about that. N. was as convinced as anyone of his own battlefield genius.
You write, "An army leaving a battle field retreating - is defeated."
No, not really. An army that leaves the battle field intact and organized is not leaving as a defeated army. The Allies made it their strategy to retire from any battle where N. was present. By this tactic, the Allies avoided having their armies destroyed and able to fight again.
You write, "I have another conclusion, he didn't calculate defeat in a battle - but realizing the weakness of morale of his army - he did not dare to construct extra bridges - otherwise it might look defaitiste - this back fired."
I suspect that a major issue was a lack of engineering resources. The main body of engineers, along with the Genie General, were off building bridges over the Saale. The French engineering reinforcements were stopped just a few miles to the east of Leipzig. This probably left Colonel Monfort and his engineering force as one of the only, if not the only, engineering force available at Leipzig. The building of the temp bridges and the mining of the bridge are both engineering tasks. N. had to make the decision which task was the most important. Clearly, to N., mining the bridge was the more essential task in order to stop any enemy pursuit. Time was essential for the French at this point. Retreating from Leipzig with the Allies right on their tail would have likely ended up with the destruction of the remains of the French Army. Better to have a portion of the army safe and intact than to have all of the army running for their lives from the Cossacks.
If the engineers were busy building temp bridges, leaving the bridge mining undone, that would have left the door open for the enemy to attack the rear of the French Army in hot pursuit.
N. was likely faced with either taking the single bridge and cutting off any pursuit or building temp bridges which would then all be used by the Allies for hot pursuit. The gamble had to be made to use the single bridge since that was the only way to keep the Army intact and safe from immediate pursuit.
And, what everyone seems to miss, if N.'s plan to use the single bridge had worked (i.e. the bridge was only blown at the last second) it would be hailed as a great tactical withdrawal in the face of overwhelming forces. According to N. (as described in Oct 20 GA bulletin) the retreat was only spoiled by the early detonation of the bridge.