Short answers, pulled from the story.
The Coalition forces prepared to assault Paris on the morning of the 30th of March and marched into the city on the 31st of March. Napoleon reached Essonnes on the 31st of March before massing troops at Fontainebleau by the 1st of April.
Joseph Bonaparte was responsible for defending Paris while Marshal Moncey led the National Guard. Marmont and Mortier reported that their defenses held until noon on the 30th of March before Joseph fled south with government officials.
Gaston Bodart asserted that the Coalition victory was won by 100,000 soldiers including 53,000 Russians, 22,000 Prussians, 15,000 Austrians, and 10,000 Württemberg and Baden troops. Digby Smith stated that the French brought 41,000 men into action against these forces.
Paris remained virtually unfortified because a plan to fortify it with earthworks was rejected by Napoleon and delayed by Joseph Bonaparte. The Senate voted to depose Napoleon and his entire family in the Acte de déchéance de l'Empereur on the 2nd of April after Talleyrand assembled a rump government.
Napoleon finally abdicated unconditionally in the Treaty of Fontainebleau on the 11th of April 1814. He had toyed with abdicating in favor of his son Napoleon II but Allied leaders vetoed that possibility before he signed the treaty.