You are very wrong

1.I know cases where people didn't surrender their I-94. Sometimes, it is the mistake of the airline personel.

2.重新入境还要凭I-94呢？ 没听说过。入境事发。
you are wrong again. To re-enter US from canada or mexico without a new visa, you have to show your old I-94 and proof of legal status (I-20 or H1b approval notice)

3.不过，既然你打算去加拿大签证，也许可以把写错的带上，要改也可以让使馆当场改。错！ 使馆不管I94
You are wrong again. If you are approved with a new visa, the embassy will issue you a new I-94 and they may or may not take away the old one.

4.不过，万一你的签证不成功或者被查，那么没有旧的-94,你想回美国就难了。 错。签证不成。就不可以回来。 没有VISA，I94能独立存在吗。

You are wrong again. Many people returned to the US after their visa applications in canada or mexico are either checked or denied. You are right about only 1 thing, techinically, it is not allowed, but in reality, many do come back with their old I-94 and I-20 or H1b approval notices. They take advantage of what is called automatic revalidation, which allows F/H visa holders to come back to the US if they are visiting canada or mexico for under 30 days even after their original entry visa is expired.

所以，你每个答案都是错的。 千万别再这样误导人了。