Actually there are two issues with the first statement - Was Will buying a car for Zach a mistake, and did he go about it the wrong way?
Was it a mistake? We don't know yet - it might very well be but we need to wait and see if Zach gets into trouble with it, as his parents and Gathan seem to think he will.
Was Will wrong in how he went about it? Hell Yes. He showed no respect for Wally and Clara's opinion on the subject, and in fact didn't even GET their opinion, but simply extrapolated what Gathan said to mean that the couple held exactly the same opinion. I live in the real world - to assume my 22 yr old daughters opinions exactly reflect mine would be a major mistake!
Am I in the crowd that thinks he didn't make any mistakes with the car? Nope. Do I think this should be an earth shattering event that gets the kid condemned by every "adult" in his life? Nope. Mistakes are made - you learn from it - you move on.
How can he learn from his "mistakes", if he doesn't see them as such? Simple - remember while it may be several weeks in our time - in Cap time this all went on in a matter of a couple days. Funeral, car, dinner making Austin uncomfortable and blow up over the car after, then off to Hawaii all in under a week.
Will may not see it as a mistake YET. Have you discovered all your mistakes in life within minutes, or even hours of having made them? Sometimes it takes a while to come back and bite you in the ass, and some mistakes bite harder than others,