The Winter Soldier was a master assassin that excelled in framing and espionage? He’s responsible for untold numbers of deaths that were made to not look like deaths. I think he could easily have made it look like an accident. Also, it is possible to be strangled in a severe car accident by your seatbelt. He could easily stage that. It’s hinted in that memory of Tony’s that Howard and Maria were possibly on their way to the Pentagon first before their flight to the Bahamas. Which makes sense, Department of Defence and all that.
The thing is, we don’t know if Bucky staged that too. He could make it look like the case broke, that the formula spilled out when the car slammed into the tree. Or more likely, someone that worked at the Dept of Defense was a secret Hydra agent and was the informant that said that Howard would be traveling with the serum. Wouldn’t be the first time Stark’s were betrayed for their creations…
Tony is actually surprisingly trusting and sentimental, especially pre-Iron Man. He would have been shattered by their deaths, he wouldn’t have questioned it. And he’s still not properly grieved them, he’s just trying his hardest not to think about it.
After the SHIELD papers leak, we’re talking billions, maybe trillions of bytes of data leaked all at once. It would take several lifetimes to sort through it all. The only reason that Zemo found a crumb of information is because he was specifically looking for anything related to the Avengers, to Tony Stark because that’s who he blamed for his family’s deaths. He saw a file with the highly publicized date of their tragic accident and started digging and happened to stumble upon all this (not easily, I mean he did have to literally find and torture a former Hydra agent and find a secret old book hidden in a wall so it wasn’t all out there for everyone to find lmao.)
I’m also assuming that most of the information, the video file especially, was all stored at the Siberian base, and thus the reason that he had to find it and lure them there. It was all kept off servers, completely isolated and self contained.
Steve did know that it was Bucky, the video that Zola shows him makes it pretty darn clear. And he knew Howard, he was friends with Howard, I think he’d have caught the news headlines and coverage that Zola showed him, he let it play for a long beat. Steve just didn’t see the video footage of it because it was locked in the Siberian facility. What I find harder to believe is that Steve isn’t more physically shocked by actually watching that video? There’s a difference between hearing about it and actually seeing your best friend kill one of your other close friends and his wife, the parents of one of your workmates. Like??? Shit man, I get choked up if I see raccoon roadkill on the highway, let alone watching the graphic deaths of people I know. But he just goes “I’m sorry, Tony. He was brainwashed.” You’ve gotta know that isn’t gonna play well with the child of the people assassinated, right? Jesus, Steve, have some tact.
Tony is the last to know because no one knew besides Hydra. The whole world, US government included, believed it was a tragic accident. Tony probably spent the last 25 years thinking his Dad got drunk at a holiday party and killed his mother and himself. That probably only served to harden his already strained relationship and view of his father. So aside from the fact that they were murdered, he just learned he spent 25 years cursing his father and blaming their deaths on him when they were assassinated. That is fucked up, my friend.