1. The history of exploring American English has been briefly reviewed.
2. The modern status of American English has been described.
Why the present perfect? I'd use the present tense here.
we have been reviewed- active
cannot allow one consider- to consider
canoe, moccasin, squaw, tomahawk, wigwam,- BTW, these have all crossed, like many more, into BrE
which are not common in more conservative British English- I have to disagree with this assessment of the state of British English. Firstly, there is a huge amount of slang in BrE and, secondly, the pace of change in language is in fact in many ways faster today in BrE, and many Americans have become the conservatives of today. The second point may be debatable in terms of degree, but there is no question that BrE is in great change, but the point about slang is just plain wrong, I'm afraid.
As a further point, you might want to look at some of the small grammatical differences, like relative pronoun usage, which don't really impede comprehension but are common and add to the tapestry of difference.
