hebrew - "Anyone who saves a life is as if he saved an entire world": Jewish life or any life?

I am puzzled by the Mishnah's original text of the famous Jewish idea that "whoever saves one life [...] saves an entire world" (Sanhedrin 4:5). The English from sefaria.org reads thus:

"It was for this reason that man was first created as one person [Adam], to teach you that anyone who destroys a life is considered by Scripture to have destroyed an entire world; and anyone who saves a life is as if he saved an entire world." And also, to promote peace among the creations, that no man would say to his friend, "My ancestors are greater than yours." And also, so that heretics will not say, "there are many rulers up in Heaven." And also, to express the grandeur of The Holy One [blessed be He]: For a man strikes many coins from the same die, and all the coins are alike. But the King, the King of Kings, The Holy One [blessed be He] strikes every man from the die of the First Man, and yet no man is quite like his friend. Therefore, every person must say, “For my sake ‎the world was created.”‎

...But the Hebrew reads thus:

לפיכך נברא אדם יחידי ללמדך. שכל המאבד נפש אחת מישראל. מעלה עליו הכתוב כאילו איבד עולם מלא. וכל המקיים נפש אחת מישראל מעלה עליו הכתוב כאילו קיים עולם מלא. ומפני שלום הבריות. שלא יאמר אדם לחבירו אבא גדול מאביך. ושלא יהו מינין אומרים הרבה רשויות בשמים. ולהגיד גדולתו של הקדוש ברוך הוא. שאדם טובע כמה מטבעות בחותם אחד וכולן דומין זה לזה. ומלך מלכי המלכים הקדוש ברוך הוא טבע כל אדם בחותמו של אדם הראשון ואין אחד מהן דומה לחבירו. לפיכך כל אחד ואחד חייב לומר בשבילי נברא העולם.

Please excuse me if there is some flaw in my understanding of the Hebrew, but it seems to me that the English omits a stipulation within the famous sentences that the life saved be Jewish ("מישראל"). Moreover, whenever I have seen this idea quoted or referenced in a non-textual source--including in shiurim given by rabbis--the "Jewish" stipulation is omitted in translation. (I even specifically asked a fifth-year yeshiva student whether this discussion referred to Jewish life or to all humanity, and he said all humanity.)

I looked a bit further and found that some Hebrew-edition Mishnaios(?) put the word "מישראל" in brackets, but all seem to contain it. So:

1) Why do the English translations leave this word out? Is it just for darchei sholom/political correctness? That seems strange (unto dishonest...)

2) Why is the word bracketed in some texts? Is there any doubt about its accuracy?

3) Why would the original text stipulate "מישראל" in the first place, considering the context? (Adam is, after all, an ancestor of "every man," as clearly acknowledged by the mishnah. How would the argument make any sense if it were limited to Jews?) [I am no longer sure about this, since having learned that a mention of "adam" in Torah usually indicates Jewry. But then someone told me that this only applies to Tanakh, or maybe Chumash.]

My best guess is that all three of these are cleared up in the commentary, but unfortunately it's way beyond me to read that...Could someone please help?

Answer

In the manuscript Parma 3173 there is no "מישראל";

In the manuscript Budapest Kaufman A50 no more;

The Mishna of Mechon Mamre, Mishna Sanhedrin 4:5 based on Rambam manuscript idem;

לפיכך נברא אדם יחידי בעולם, ללמד שכל המאבד נפש אחת, מעלים עליו כאילו איבד עולם מלא; וכל המקיים נפש אחת, מעלים עליו כאילו קיים עולם מלא. ‏

In Shinuye Nussachaot Shas Mishnayot Vilna, the divergence between versions is signaled.

So the translation in English is not so strange, may be based on one of the above cited manuscripts.