So Smith knows of only two races that have inhabited the country before his time.
How does that preclude the existence of others?
Those responsible for the 'first settlement' were the Jaredites, who were eventually destroyed. The rest were clearly (following the Book of Mormon) the descendants of Nephi's party, whose 'principal nation' (from the Book of Mormon , the Nephites) were wiped out by (from the Book of Mormon) the other nation, the Lamanites, at which point, as the Book of Mormon tells us, consistent with the angel's account, "the blessings of God [were] finally withdrawn from them as a people". The Lamanite 'remnant' are the Indians.
The context of the first settlement is only that the the Book of Mormon peoples. Again, other inhabitants not precluded.
I just don't see:
(a) Where the doubt comes from about the Jaredites being the first people in America.
What about pre-flood (local or global)? That alone provides context that destroys your argument.
Heck, it was the Jaredites who were, according to Smith, responsible for the 'first settlement'. He had two sources for that fact, one the Book of Mormon and the other the account by the angel, which included, he says information on the origins of the aborigines.
Those particular aborigines yes.
There is no sign that Smith felt his sources were inconsistent - which would have been very remarkable, given the almost certain identification of the angel with Moroni, the last premodern person to possess and read the Book of Mormon.
Non sequitur
(b) Why bcspace would find uncongenial the very obvious conclusion that Smith believed and stated that the Jaredites were the first people in America.
What if he did? How does that preclude the existence of others? Neither the Book of Mormon, Joseph Smith, or the angel state anything of that kind.