That's what makes a good retcon. Obi-Wan's dialogue about Anakin and Vader in ANH was meant to be true when it was written, but it still works with the post-Empire story because it makes complete sense that Obi-Wan would want to hide the truth of Luke's heritage from him. Even the way it's acted fits perfectly, with Alec Guiness visibly hesitating before telling the lie. It's so smooth that it's completely believable that it was always the original intention, even though it wasn't.

Where retcons become problematic is when you just have to straight up ignore details that don't fit the later version because they don't make sense anymore.

Doesn't really matter if the cue itself is canon, because Obi-Wan's reaction is. I only brought up the cue to show that, even in the 70s when the movie was being written, the intent of the scene was that Han was speaking nonsense.

It is kinda weird that they use Parsec specifically, but Parsecs are a thing that exist in Star Wars (Padme uses the same word in AotC, referring to a distance). That's also a different problem from the one you originally outlined (that Han uses a measurement of distance instead of time).

But I basically just see it as translation convention. The characters use an Earth-based word because the film has them speaking in an Earth-based language, even though the actual in-universe language of Star Wars is Galactic Basic, not English.

Because he's wrong!

That wasn't a goof, people just misunderstood the scene. Han isn't bragging about his ship, he's testing Luke and Obi-Wan to see if they know anything about space travel (because they'll be easier to swindle if they don't).

If you watch Obi-Wan you can tell he sees straight through it. The shooting script actually includes the line "Ben reacts to Solo's stupid attempt to impress them with obvious misinformation."

But people took Han's claim at face value for some reason until the Kessel Run being distance-based became an actual thing.