talmud bavli - To what extent is someone on the way to do a mitzvah protected from danger?

Pesachim 8A (link) says:

R. Elazar taught that harm will not befall a Shali'ach (someone on the way to do a) Mitzvah!

That is a short excerpt from a discussion about being protected when doing a mitzvah in the context of searching for chametz before Pesach where there might be a danger. It seems to say that if someone is going to do a mitzvah, unless there is something especially dangerous, one is protected from harm.

Of what degree of protection does it speak? Protection from death? Major injury that wouldn't heal (loss of limb)? Major injury that doesn't have a major long term effect (hip fracture that is set but has residual minor long-term effects)? Temporary injury (broken toe that gets set and fully healed)? Even temporary minor injury (paper cut)?

Answer

From the ברייתא cited further in the Gemara, it would appear that any and all damages, even of the slightest and likeliest nature and occurring anywhere on one's property are included in this discussion (at least according to the opinion voiced there, although there is no contradictory sentiment earlier in the discussion that I've detected) [text of Rashi follows that of the Gemara]:

תניא: איסי בן יהודה אומר: כלפי שאמרה תורה "ולא יחמד איש את ארצך", מלמד שתהא פרתך רועה באפר ואין חיה מזיקתה, תרנגולתך מנקרת באשפה ואין חולדה מזיקתה; והלא דברים קל וחומר, ומה אלו שדרכן לזוק אינן ניזוקין, בני אדם שאין דרכן לזוק על אחת כמה וכמה; אין לי אלא בהליכה, בחזרה מנין, תלמוד לומר: "ופנית בבקר והלכת לאהליך", מלמד שתלך ותמצא אהלך בשלום.

כלפי שאמרה תורה: מתוך שאמרה תורה כך, אנו למדים שהבטיחו הכתוב שלא יוזק ממונו, וכל שכן גופו ד*אין דרכ*ו לזוק, דאדם אית ליה מזלא ואינו מהיר להיות ניזק בגופו.

To quote the outline on the page you provided in your question:

(Beraisa - Isi ben Yehudah): "V'Lo Yachmod Ish Es Artzecha" teaches that one [who goes to Yerushalayim for the festival] may leave his cow grazing in the dirt and his chicken pecking in the wasteheap, and Chayos and weasels will not attack them;

It is normal for animals to be damaged, yet the Torah promises that they will not be [due to the Mitzvah]. All the more so people, who are not normally damaged [because they have Mazel, i.e. supernatural protection], will not be harmed!

Question: This teaches that they will not be harmed on the way. What is the source regarding the way back?

Answer: "U'Fanisa va'Boker v'Halachta l'Ohalecha" - you will return and find your tent b'Shalom (intact).