I'm very curious to know...how often is the narrative driven by the actions of the PCs? How often do decisions players make actually effect events that happen in the game?
The structure I'm providing is more "narrow-broad-narrow". The PC's are bounty hunters. Right now I am not giving them really the ability to choose their own jobs. The standard structure is the Guildmaster tells them there is a particular job he wants them to take and the job involves acquiring some bounty with some stipulations. So at that level, the PC's don't have a lot of choice about what to do. This isn't sandbox through the Star Wars universe where the players just decide what planet they want to hyperspace off too, access a list of thousands of bounties, and pick the ones they want to pursue. I can imagine such a game, but not running it at any point after becoming an adult. That full sandbox play takes more work than what I do.
However, within the context of "this is the job" the players can do pretty much whatever they like, and while I have ideas about what they are most likely going to want to do, they often surprise me and do things I hadn't anticipated or prepared for. How the story turns out is up to the players. For the most part, the group seems to not invest to much in which faction is winning as long as their crew is winning and they get paid. They have avoided sacrificing their interests to advance a particular faction's interests, and they generally work for whomever is offering the most pay. I think a different group might end up making different choices, and I have at least run the first adventure for two groups and they played things out entirely differently.
I should note importantly that this game is what the players asked for. It started shortly after The Mandalorian came out and we were talking about next campaigns after winding down a CoC campaign that I don't think anyone was interested in continuing (I've decided CoC isn't really to the tastes of this group) and "Let's all be Star Wars bounty hunters!" was the players unanimous choice.
Do players just play through all the stuff that's prepped? Can players in your games completely abandon a prepared plotline, like forever?
Well, if they don't acquire the target they don't get paid, so they have incentive in game to not just abandon a job. The Guildmaster is generally on their side, but things might not be as pleasant if he wasn't because they abandoned time critical jobs with big payouts. They have a "boss".
The did abandon a plotline once after finishing the job where I had prepared a second job with in the same story. They instead chose to bugger off and fight it out in court.
How often do you manipulate events within the narrative to ensure that PCs engage with the scenes and encounters you have prepared?
Almost never. I can't think of a time I've done so in the Star Wars game. The only time I do this really is when the players are "stuck" and can't figure out what to do and aren't pushing forward and I can tell the players are getting dispirited, I will generally push something in front of the path of the players to help them get unstuck (like an ambush or stumbling on a crime in progress or the bad guys taking some plausible action that comes to their attention).
Do you regularly abandon thousands of words of prepared material if the players decide not to engage with what you have prepared?
Yes. I rarely use all the prepared material because the players rarely push every lead or go to every location that I think they might try. For example, in this current adventure they aren't going to end up investigating the Smashball Ultras, they never went to the Smuggler's bar to try to come at the problem from the black market weapons trade, they didn't find either of the two safehouses in time to have a shootout at those locations, and while they visited the racing teams custom swoop shop, they didn't really snoop around the way I expected they would.
Can players choose to ignore plotlines you have prepared?
Players could certainly go, "I'm not going to take this job." But in general, players don't do that. For one, I think I have a reasonable amount of trust with my players that I'm not going to write an adventure that is just going to screw them over more than not doing the job would. And secondly, because well most of the time in the 40 years of playing RPGs, players generally agree to play the adventure. Players know they need to "bite the hook" if they want to find the fun, and that if they turn down the adventure that it's not at all guaranteed there will be something equally fun to do. The idea that most players want to come up with their own fun is frankly bizarre. I know I don't as a player. The reality of trying to come up with your own fun is likely to be a grind. The real world just isn't filled with fun and any simulation of a setting just isn't going to have fun everywhere. At least my version of "Paychecks & Paperwork" involves exciting narratives, high stakes adventure, and big splashy payoffs in fame and fortune.
I'll be quite frank though. You wrote a whole paragraph of questions that were just variations on, "Are you really such a bad GM as I imagine?" Someone said he wasn't calling me a liar and then did. You just wrote in essence, "I'm not saying you run a railroad but also I am."