Short answers, pulled from the story.
Prussian Field Marshal Gebhard Leberecht von Blücher commanded the Allied Army of Silesia while Austrian field marshal Karl Philipp, Prince of Schwarzenberg led the Army of Bohemia. The specific battle involved Russian corps under General Mikhail Sacken and Prussian divisions led by Yorck.
Napoleon ordered Étienne Marie Antoine Champion de Nansouty to capture Montmirail with two cavalry divisions followed by Ricard's division of Marmont's corps. He subsequently directed Ney's Young Guard divisions and Mortier's Old Guard to march directly from Sézanne to reinforce the attack.
The primary engagement took place west of the village of L'Épine-aux-Bois where Lieven's corps held a position south of the highway. Fighting also extended east toward Marchais and Fontenelle-en-Brie along the Château-Thierry highway.
Allied losses ranged from a low of 1,500 Russians and 877 Prussians to a high of 3,000 killed and wounded plus 708 prisoners. Specific reports indicate the 1st Prussian Brigade lost 877 officers and men while Sacken's troops eventually reached Viffort after an all-night march.
Blücher lacked a liaison officer with Seslavin and was unaware that Schwarzenberg had switched Seslavin's force to the extreme left flank without informing him. This communication gap created a dangerous void on the Prussian left side before the battle commenced.