Depends on the person, but in general, back squats are going to show many more problems than front squats. You are sitting into your posterior chain on a back squat and most people dont have one. You can look pretty good on a front squat and it can be complete shit. It can take a pretty good eye to spot problems there sometimes.
But back squats can look better, it depends on the person.
As far as heavy loads go. There is going to be a point where EVERYONE will start to fall apart. It just depends on where you want to call it. Ideally you would maintain close to perfect form until you hit a weight that couldn't move, but even then your form would be fine you just couldnt move it. You see this alot in top level olympic lifters and powerlifters. They get stuck but they will not waiver on form much.
Stop over thinking it. You need to get a good corrective exercise program and go from there. The issue is if you can't squat properly with a weight that doesnt even challenge your strength. Which is most people. Then they just keeping adding weight to the bar.