Question: What was so wrong with Marty and Doc returning to the future and stopping Old Biff from stealing the DeLorean? If they stopped Old Biff from stealing it, he wouldn't be able to go back in time to give his younger self the almanac.
Answer: As Doc explained, they'd be traveling to an alternate future. A future where old Biff already has the almanac, and thus, the alternate old Biff wouldn't have a need to steal the DeLorean to give his younger self the almanac. (I believe there've been discussions on here about why did old Biff then return to the same future Doc and Marty were in instead of the alternate future, and it's implied he basically fades out of existence.)
Answer: To use Doc's example in the film, think of time as a linear straight line. Beginning at the point where the past was changed (i.e. Old Biff giving the almanac to his younger self in 1955), time would then continue linearly; therefore, traveling to the future from 1985-A would result in visiting the future of 1985-A. That's why Doc and Marty had to go back to 1955 to get the almanac back; once the anomaly was corrected (i.e. the almanac removed from where it didn't belong), the 1985-A timeline would vanish and time would resume its course the way it should in the original 1985 timeline.
Question: When the police pick Jennifer up and start driving her home, Doc says the Skyway is jammed and it's gonna take them forever to get there. 1) Wouldn't the police be sat in the same traffic? Taking Jennifer home isn't an emergency, so they wouldn't be using lights and sirens. 2) If the Skyway is jammed, why not just do it the old fashioned way and drive to Hilldale rather than fly?
Question: When Marty is walking through 2015 Hill Valley, he sees a billboard from Goldie Wilson III in which he is offering a hover conversion from $39,999,95. Where did Doc get the money from to have the DeLorean converted in the first place? This was also the starting price of a hover conversion, it may have even been more than $40,000 depending on how complex it was to fit it to the DeLorean.
Question: Was there any particular reason as to why the taxi cab in 2015 was built upon a Citroën DS? It would've been 60 years old in 2015, so it doesn't scream "futuristic car".
Answer: In real life, it was because the DS looked similar to a drawing one of the artists had come up with. But it does have a unique and futuristic look to it since it wasn't really a popular car or seen that often in the 80s in the UK. Since it wasn't meant to be the star of the show, they built off a working, existing car rather than design and build a working, unique car. If you're talking about in the Back to the Future universe, the taxi was meant to be a brand new model. It's even possible the taxi company retrofitted 60-year-old cars the same way people build hot rods out of old cars. Here's an inside look at the taxi. https://m.youtube.com/watch?v=Cb9Gtd-ycto.
Question: Why was Crispin Glover replaced by Jeffrey Weissman?
Answer: A contract dispute. Glover wanted more money, especially for two movies, but producers thought why pay more for what were essentially cameos.
Answer: Relates to the "branching timeline" interpretation of time travel. Basically they jump forwards to a future version of the timeline where they never left. When they return they make different choices and that branch doesn't happen any more, taking them on a new path. Alternatively (additionally), given that they do eventually return, that's all that matters. If they left somewhere at 10:00am, had time travelling adventures, but then returned at precisely the moment they left, at any point later they're still there like they never went anywhere.