Short answers, pulled from the story.
The Royal Albert Hall opened on the 29th of March 1871, brought forward from the originally planned date of the 1st of May, the twentieth anniversary of the Great Exhibition. Queen Victoria was too overcome to speak at the ceremony; the welcoming speech was delivered by Edward, the Prince of Wales.
Queen Victoria renamed the hall in memory of her husband, Prince Albert, who died in 1861 before his vision for a public cultural precinct in South Kensington was realised. She changed the name from the Central Hall of Arts and Sciences to the Royal Albert Hall of Arts and Sciences when she laid the foundation stone on the 20th of May 1867.
The Royal Albert Hall has a seating capacity of 5,272, including standing in the Gallery. The hall was originally designed for 8,000 people and has at times accommodated as many as 12,000, but present-day safety restrictions limit it to 5,272.
The echo was caused by the hall's large wrought-iron and glazed dome, which sent sound bouncing back to the stage. A canvas awning was suspended below the dome after the opening in 1871 to reduce the effect, but the acoustics were not properly solved until 1969, when large fibreglass acoustic diffusing discs were installed beneath the ceiling. These discs became known as the "mushrooms" or "flying saucers."
The BBC Proms moved to the Royal Albert Hall in 1941 after the Queen's Hall was destroyed in an air raid. The Proms had originally been founded in 1895. The concerts have been held at the Royal Albert Hall every summer since the end of World War II.
Eric Clapton has performed at the Royal Albert Hall over 200 times since his debut there with the Yardbirds in December 1964, passing the 200-show mark in 2015. James Last appeared 90 times at the hall between 1973 and 2015, making him the most frequent non-British performer to have played the venue.