Wa alaikum assalam brother,
Please don't be confused or worried. What have believers to worry about? That a messenger might come into our midst or someone might come to present clear messages of the scripture to others? Why should that be a cause for worry other than such a belief may conflict with what we have been taught from childhood?
After all, we must be prepared to put ‘confirmation bias’ aside (the tendency of individuals to prefer or seek out information that confirms their existing beliefs).
Ultimately, it is not important what I believe, or want to believe, or what you may respectfully believe or want to believe. As I trust you will appreciate, we are only to derive the best arguments from the Quran. If the Quran says messengers will come then who are we to argue? As you read in the article, verses are quite clear in what they state.
007:035
"O ye Children of Adam! whenever there come to you messengers from among you, rehearsing My verses (Arabic: ayati) to you, those who are righteous and mend (their lives), on them shall be no fear nor shall they grieve"
I can't comment on '19ers' or 'Ahmadi's' as I am neither of these groups. I am simply a student of the Quran and disagree on many points with these groups. Prophets do come with books and or authority or even confirm a Book in their midst (like prophet Aaron aiding prophet Moses with authority).
It is also incorrect to say that the Jews only had the Torah. The 'Tanakh' which is the canon of the Hebrew 'Bible' consists of the 'Torah, Nevi'im and Ketuvim. The Nev'im (Prophets) includes books such as Joshua, Samuel, Isaiah, Jeremiah, Amos, Jonah, Zechariah and others. The Quran implicitly confirms this.
003:084
Say: "We believe in God, and in what has been revealed to us and what was revealed to Abraham, Isma'il, Isaac, Jacob, and the Tribes, and what was given to Moses, Jesus, AND THE PROPHETS, from their Lord: We make no distinction between one and another among them, and to Him we are submissive"
The Psalms given to David is mentioned separately (4:163)
The Quran also shows distinction between the scrolls of Moses and that of Abraham (87:19) implying that the latter document may have also been in possession of the 7th century community.
Therefore dear brother, please do not be alarmed. Just assess the verses that have been shared and allow them to speak to you with an open mind.
Only God knows how many 'ordinary humans' have passed on the 'message' to their communities all over the world at different time and how many will continue to do so in their capacity as message bearers.
010:047
"And for every nation there is a Messenger. And when their Messenger comes (on the Day of Judgment) it will be judged between them fairly, and they will not be wronged"
I hope this helps, God willing
Joseph