Past Perfect and Past Simple in AmE

I've recently been told that Past Perfect and Past Simple tenses are interchangeable in AmE. I know that it is possible to use the Past Simple tense instead of the Present Perfect one in AmE but have never faced the use of the Past Simple tense instead of the Past Perfect one.

I've been provided with the following examples:

I have already typed 10 pages of my report (Present Perfect, typical for BrE)

I already typed 10 pages of my report (Past Simple, typical for AmE)

I had typed ten pages of my report by the time he returned. (Past Perfect, typical for BrE)

I typed ten pages of my report by the time he returned (Past Simple, typical for Ame).

Are these uses correct?

As an American English speaker, all four examples seem correct.

The past perfect often seems to be used in American English to avoid confusion about the relationship between two past events. Where such confusion is impossible, the simple past is often used.

"I had typed ten pages when he returned." indicates typing before returning. "I typed ten pages when he returned." implies that returning is simultaneous or even before the typing.

In your examples, "by the time" serves this temporal ordering purpose, so the choice of verb tense is unnecessary.