Question: Paul Ruebens does the voice of Max but why in the credits does it say that the voice of Max was done by Paul Mall?
Answer: It was Paul Reubens' idea to use a pseudonym, so as to keep the voice of Max a mystery and to surprise any fans of his.
Question: Even though it's not technically indicated in the film, I always wondered if the young blonde lady watching the spaceship shown for a few seconds in 1986 just before David returned to 1978 was Jennifer (the 12-year-old girl David had a crush on in 1978). She looks roughly 20 years old now herself and I thought that could be her just to show what she looks like now (maybe there was a deleted scene with her in it). Was this a correct assumption?
Answer: I can't directly answer this, but I was watching this movie just now, and didn't take in the girl at the beginning's (Jennifer's) name, so when Sarah Jessica Parker, Carolyn, appeared I then spent the rest of the movie assuming that she was going to turn out to be the girl he was crushing on in 1978, especially when Carolyn even told him he was cute (which is kind of a weird thing for a 20-year old to say to a 12-year-old if there isn't a plot reason for it). I do wonder if that was in fact the originally intended idea that was changed for whatever reason before the movie was completed, and if perhaps he was meant to go back to 1978 with the knowledge that she found him cute and therefore a new-found confidence to ask 12 year-old her out.
Answer: The only people with different young and old actors were Jeff and Carolyn. The scene with young Carolyn would have been after he got back from the future and was filmed, but cut for pacing reasons.
Question: Who actually sent RALF to take David to the ship? The ship tells him to get in RALF but later on we see Carolyn looking shifty when they're questioning how he escaped?
Answer: Carolyn wouldn't have wanted David anywhere near the UFO as it could've been dangerous, so it's safe to assume that "Max" programmed the R.A.L.F robot. Although Carolyn didn't have anything to do with with David's escape, it wouldn't have escaped unnoticed that the R.A.L.F robot was in the hanger where the UFO was. Given that she's always the one with the R.A.L.F. robot, it's safe to assume she was acting worried that she'd be blamed rather than acting "shifty"
Answer: According to IMDB, Paul Mall is just another one of his names people know him by.
Sir William