1879 would make it a Morgan dollar, which has an eagle on the reverse.
Yes. That's a Morgan silver dollar.
The silver content is given as a "fineness" on the coin's reverse side. It should show something like ".999" which would mean 99.9% pure silver.
1899 would make it a Morgan dollar. The mint mark is located on the reverse (tails) side, near the bottom, just above the letters "do" in the word "dollar."
1928 would make it a Peace dollar. The mint mark is located on the reverse (tails) side, just below the word "one."
1923 would make it a Peace dollar, so the mint mark is located on the reverse (tails) side, just below the word "one."
On the reverse side between the letters and the eagle
The date 1891 would make it a Morgan dollar, so the mint mark is on the reverse (tails) side, near the bottom, just above the letters "do" in the word "dollar."
"DO" isn't a mint mark. The mint mark on a silver dollar for that year is on the reverse (tails) side, just above the letters "do" in the word "dollar."
1880 would probably make it a Morgan dollar, so the mint mark (assuming that's the marking you meant) is located on the reverse (tails) side, near the bottom, just above the letters "do" in the word "dollar."
1882 would make it a Morgan dollar, so the mint mark is on the reverse (tails) side, near the bottom, just above the letters "do" in the word "dollar."
Looking at the front of the coin, the mintmark is just above SBA's shoulder, on the left side.