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".
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.
Question: Does the trilogy stick to a coherent time-travel-logic or is it "mix-and-match"? While it purports to adhere to the "one universe, many detours" theory (which is why Jennifer is save in bad 1985), it also delivers proof for the multiverse theory, unless it's "explained away" such as: Doc was never killed. He already wore a vest (and brought a gun to the meeting with a teenager) because he was a bit paranoid. Since he never really died, there's no parallel timeline required for him to stay dead.
Answer: It's fairly consistent. Changes to the past affect the future, although the time travellers themselves are afforded a bit of convenient wriggle room, like time changing around them, changes not immediately taking effect, etc, so as ever some suspension of disbelief is needed. The timeline changes - originally Doc was killed, Marty went back, gave him a letter, Doc took precautions. That's not the multiverse, that's just the future being changed by actions in the past.
Answer: Why wouldn't they exist? This is a serious question (maybe I am missing something). In BttF, Marty was disappearing because his parents weren't going to get together for him to even exist. In BttF II, his parents got together (Biff says so - he calls George Marty's father) and he was born, so it is very consistent between both movies. So even with an erased timeline, Biff did not erase Marty and his siblings being born. As for the linear time or multi universal, I think the movie is consistent - only the time traveler remembers things that happened before the time traveling began.
Answer: It's completely mix-and-match IMHO. The movies constantly switch between linear and parallel timelines, either making changes affect the time traveller or not, depending on plot convenience. For example, in the first movie Marty is in danger of disappearing unless he gets his parents back together, and fixes it before undoing all he had done himself, which causes a paradox. But then, when he gets back, his parents and siblings are completely different, but Marty is the same person that supposed lived that new life, unreplaced. That simply doesn't make sense in a linear timeline. In the second movie it is even worse, with Marty and Doc still existing in a timeline erased by Old Biff with the sports almanac, for plot convenience.
Question: The mechanic tells Biff the price of Biff's car is $300. Does that include the damage Biff caused to the truck he hit if the first film and also the price of the horse manure that had to be hauled away? I know Biff goes into a store across the street for a few seconds so we don't hear part of the argument. Wouldn't Biff have to pay more than $300, or was the $300 just for the car itself?
Answer: Remember that Biff's car was going very slowly and hit side on to the back of a much larger truck. It didn't damage the truck, it only caused the manure to come out.
Answer: The mechanic does mention the horse manure in the dialog of the $300 but truck is not mentioned so it's unknown.
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.
Bishop73