Short answers, pulled from the story.
The Peninsula campaign began on the 17th of March 1862 when an armada carrying 121,500 men departed Alexandria. This force included thousands of horses and dwarfed all previous American expeditions in scale.
Confederate General Joseph E. Johnston was wounded by an artillery shell fragment at dusk on the 31st of May 1862, and evacuated to Richmond. He did not die from this wound but G.W. Smith assumed temporary command after his injury.
The USS Galena took 45 hits while remaining almost stationary for over three hours near Fort Darling. The vessel suffered casualties of 14 dead or mortally wounded and 10 injured before withdrawing to City Point around 11 a.m.
McClellan believed he was outnumbered two to one due to faulty intelligence provided by detective Allan Pinkerton. This misinformation caused him to misjudge the actual strength of Confederate forces throughout the campaign.
Robert E Lee launched fierce counterattacks known as the Seven Days Battles starting on the 25th of June 1862. These attacks continued through July 1, forcing the Union army into retreat despite none being significant Confederate tactical victories.