What connection is there, if any, between Biff's return and Willy's inability to get past Yonkers?
It would be difficult to prove there is any direct connection. Willie has been having a tough time driving for a while, and this may be just another example. However, it's always interesting to try to establish why he included the specific incident and why Yonkers. It is safe to assume, I think, that Willie lived in Brooklyn (based on the reference to Ebetts Field). Yonkers is just above the county line in the Bronx. That tells us that Willie didn't get very far, just outside his "home" zone. It also tells us that, if it took him hours to drive home, he was driving very slowly.
I sounds like this may not be the first time this has happened to Willie, and that leads us to consider why this happened. It is quite possibe that this is linked to the added stress of having Biff home. Biff shares Willie's secrets of his infidelities; it is probably safe to assume that after all these years Bif is not going to reveal his secret, but it's always something to be concerned about. Perhaps more importantly, Willie knows that Biff's "failure" is directly connected to his own infidelities. Knowing that Biff was going to be there meant that he would have to deal with these realities again, and may have caused greater than normal stress for him. So that may be the connection ....
Willy is upset because Biff is coming home, yet again, unable to hold down a job or make it on his own. While Biff isn't entirely incompetent, the problem is that his attitude and puffed up sense of self importance prevent him from staying employed for very long. In fact, he averages a new job every six months... hardly enough time to start a career and build a steady work history.
This inability on Biff's part to achieve a career goal is also directly related to Willy's inability to get past being "the New England man" at his sales job. Willy is having increasingly difficult times on the job, almost wrecking his car more than one time on the drive home. Additionally, he forgets where he is, and drives several hours out of the way before figuring out which way he needs to go. His mind is slipping, but unfortunately, so is his work performance, which prevents him from being able to earn a non-travelling job close to home.