No need to be disturbed.
First of all, the human authorship is less important than the Divine authorship.
Second, the Holy Spirit can inspire additions to texts, as well. For example, there's no denying that Moses isn't the author of the last chapter of the Torah: it's Deuteronomy 34, and it's about Moses' death. Yet it's always been accepted as part of inspired Scripture, by even the most conservative of Jewish factions (the Sadducees, who had only the first five Books in their Bible).
Third, many of these "additions" may be "subtractions." For example, Luke 22:43-44 is reported from a very early date. It may well have been simply omitted in one of the translations because a copyist skipped a line.
Fourth, the material in these "additions" is ancient in origin. Nobody is claiming that these are Medieval additions. For example, the doxology at the end of the Lord's Prayer (Matthew 6:13) is attested to in the Didache, which is as old as the New Testament.
Fifth, all of the controverted sections are orthodox. None teach anything contrary to the rest of Scripture: in fact, whether originally written by the human authors, these statements all seem to be true.
Sixth, as Catholics, we don't believe in sola Scirptura. Even if the entire Bible were lost tomorrow, we could know the Gospel through extra-Scriptural attestation from a very early period.
Seventh, there's no reason to hang your faith on this one way or the other. What difference should it make for your immortal soul whether or not the pool of Siloam was stirred by an angel or not?
To the extent that additions exist, most of them are from what are called glosses. The idea is this. An early Christian wrote notes in his copy of, say, the Gospel of Luke. When another Christian went to copy his personal Gospel of Luke, the notes got included along with the main text. No conspiracy, no apparently intentional Bible-tampering, just an innocent mistake.