The ladder led to a painting hanging from the ceiling. Chained to the painting was a magnifying glass, and when Lennon looked at the painting through the glass, he saw the word YES in tiny letters. He was impressed by the positive message and made it a point to meet Yoko Ono.
Yes, in the back room of yokos.
We don't know, and we will probably never know cuz everything was black and white in there time :)
There are multiple Yokos across anime, but the most well-known would probably be Yoko of Tengen Toppa Gurren Lagann. (Her last name is either Rittona or Littner, depending on the translation. It's most likely spelled Littner but pronouced Rittona by the Japanese).