1- You can go to matchgame.org to see the Password Plus celeb listings, which will tell you when the rule change took place. On the previous versions of Password, the person that didn't get the previous word got the first clue/option. They realized later that this gave the opposing team an unfair advantage, because they'd have extra clues to help them solve the puzzle.
2- I believe the play/pass option started in the 70s run of Password
3- The passwords were loaded onto a carousel, and each puzzle and password was color coded. The producers would know if Allen had the carosel turned to the appropriate color. They later installed a small camera in Allen's podium, and covered up the rectangle. One thing I did notice about the desk was that the blue area was "hollowed out", and in later episodes, it's a solid blue rectangle. Why this is so.. I dunno.... Chris?!
Other things to note about the premiere week...
According to Chris C, the Alphabetics board was always intended to be placed behind the grey wall, but the doors weren't ready.
They experimented with how the players and celebs should stand or sit during the credits. During the early episodes, you'd see Allen, the contestants and the celebs standing around, and moving over to the Alphabetics position. Also, they had fun with Allen's "pit" as they went up and down the steps. (Later in the run, a contestant would fall into Allen's pit after being so excited about winning the big money)
Sometime later in the run, neon was added to the puzzleboard, and later still, the puzzleboard got the blue checkerboard motif, which Allen first said he disliked, but he later mentioned that he warmed up to it.