Ka-Bella, that article in the Catholic Mirror is authentic. There are numerous sources of "Rome's Challenge" all over the web and none challenge its authenticity. Indeed, the references made in the article are historical events (like the boycott of the World's Fair that led to it's publication). Gibbons (1834-1921) referred to also existed as a Catholic official at the time.
http://en.wikipedia.org/wiki/James_Gibbons.
It is not surprising that you are shocked by what the articles say. Catholic doctrine on the Sabbath has shifted with time. At the time of writing the article (they are three of them in Rome's Challenge), the Catholic church spoke with certainty that the Sabbath was changed. The church no longer referred to Saturday as the Sabbath but to Sunday. Pope John Paul II referred to Sunday as the Sabbath numerous times in his encyclicals. Popes after him do the same. Trace the change from the Nicene Council (320s) and the Roman emperor who joined Christians at that time.
So what was the point of writing the article in the 1830s? It was to demonstrate the hypocrisy of the protestants who reject papal authority while imbibing in its Sunday doctrine. The Catholic church clearly says that without scriptural authority, it transferred solemnity from Saturday to Sunday. The point was that the Catholic church has authority to change God's law (erroneous of course), and sola scriptura is a strawman because Protestants accept Sunday worship which is nowhere mandated in the Bible. The Catholic Mirror shows that contrarywise, Saturday is the Sabbath and has remained so over the course of time, as demonstrated by the unbroken link of Jewish Sabbath keeping.
Many Catholics do not know that their church changed the Sabbath from Saturday to Sunday. Many protestants are also in ignorance of that fact.
As to what the early church fathers believed or practiced, a lot can be said. Apostles kept Saturday as the Sabbath (the Catholic Mirror articles state as much, and they quote several verses). The popular adoption of Sunday as the Lord's Day was at first a reference to the resurrection and then to John's vision (which again the Catholic Mirror rightly shows is erroneous). Those who oppose the keeping of the Sabbath commandment have drawn all sorts of defenses but the Catholic Mirror article shows the folly with solid scriptural backing.
If you follow the posts here, no scriptural backing for Sunday worship is offered except the spurious claim that the Lord's Day is Sunday. The Catholic Mirror thoroughly debunks that claim. There is simply no scriptural backing.
In effect, having changed the Sabbath from Saturday to Sunday, the Catholic church popularized Sunday worship. At some point in history, people worshiped on Saturday and Sunday. Afterwards, they shifted the debate and started calling Sunday the Sabbath. That is the position that retains to date.
The articles in the Catholic Mirror show there was no confusion about what the true Sabbath was. It refutes your argument in the last para about Jews and the formal marking of the Sabbath.
Adventists have ample proof of the sanctity of the Sabbath (Saturday) and the falsity of Sunday worship. We quote Catholic documents like the Catholic Mirror to avoid unnecessary controversy over sources and authenticity.
Google "Who Changed the Sabbath Day" for more answers. One of them is here.
http://www.sabbathtruth.com/free-resources/article-library/id/916/catholic-church-admits-they-made-the-change