shluchei mitzvah ainan nizokin

The gemara Pesachin 8b is the source for the concept of shluchei mitzvah ainan nizokin based on a pasuk at the end of our parsha (34:28):

א"ר אלעזר שלוחי מצוה אינן ניזוקין לא בהליכתן ולא בחזירתן כמאן כי האי תנא דתניא איסי בן יהודה אומר כלפי שאמרה תורה ולא יחמוד איש את ארצך מלמד שתהא פרתך רועה באפר ואין חיה מזיקתה תרנגולתך מנקרת באשפה ואין חולדה מזיקתה והלא דברים ק"ו 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 וכי מאחר דאפילו בחזירה בהליכה למה לי לכדר' אמי דא"ר אמי כל אדם שיש לו קרקע עולה לרגל ושאין לו קרקע אין עולה לרגל 

The Noda b'Yehudah (mh"t OC 94) asks why it is that the Rambam does not quote R' Ami's din that someone who does not own land is exempt from aliya la'regel. (Parenthetically, Tos Pesachim 3b d"h alya seems to compare the chiyuv to bring korban pesach with aliya la'regel and exempt someone who does not have land.) This is the sugya's conclusion and there is no dissenting opinion?  He answers by quoting the gemara a few lines earlier on the page:

בעו מיניה מרב הני בני בי רב דדיירי בבאגא מהו למיתי קדמא וחשוכא לבי רב אמר להו ניתו עלי ועל צוארי ניזיל מאי אמר להו לא ידענא

Rav told his students that they could rely on not coming to harm travelling in the dark to get to yeshiva, but he was not sure whether the same rule applied for the trip home.  

Why was Rav unsure of what would happen on the way home?  Rav apparently held that the rule of shluichei mitzvah ainin nizokin applied on the way to a mitzvah, but not on the way from.  Meaning, he must have learned the pasuk in our parsha לא יחמוד איש את ארצך as speaking only about travel to a mitzvah, not from, in which case there is no kal v'chomer, and the din of R' Ami has no foundation.

The Oznayim laTorah rejects this approach.  Rav had a safeik, he writes, not because he rejected the kal v'chomer, but rather because the mitzvah of talmud torah is not the same as the mitzvah of aliya la'regel. When it comes to aliya la'regel, after 7 days the mitvah is done.  In that case, the Torah guarantees that you will return home safely.  When a person goes to the beis medrash to learn Torah, at what point can we really say the mitzvah is done?  (The Sefer haChinuch #430  discusses why mi'doraysa we recite a bracha *after* eating food, but birchas haTorah *before* learning.  Perhaps the answer is that there is no end to learning, so at what could Chazal be kove'a the bracha?  See the Chinuch's answer.)  Who can say with clear conscience that they are not going home to relax and do something else, or because they are bored, or because they are tired of their chavrusa, etc.?  The guarantee of  אינן ניזוקין לא בהליכתן ולא בחזירתן applies when the mitzvah is over, not if you abandon the mitzvah in the middle.  Therefore,  Rav was not prepared to say that the protection extended on the way home from the beis medrash.

I am not sure this next part makes sense, but I will throw it out there and make of it what you will.  I want to sharpen the chiluk of the Oznayin laTorah.  The sugya in Pesachim adds one important qualifier to the rule of shluchei mitzvah ainan nizokin: ן הֵיכָא דִּשְׁכִיחַ הֶיזֵּיקָא שָׁאנֵי  When there is a real and present danger, all bets are off.

Kesubos 77b:

מכריז רבי יוחנן הזהרו מזבובי של בעלי ראתן רבי זירא לא הוה יתיב בזיקיה רבי אלעזר לא עייל באהליה רבי אמי ורבי אסי לא הוו אכלי מביעי דההיא מבואה ריב"ל מיכרך בהו ועסיק בתורה אמר אילת אהבים ויעלת חן אם חן מעלה על לומדיה אגוני לא מגנא 

Even though being near people afflicted with the sickness called raasan was dangerous, R' Yehoshua ben Levi would sit and learn with them.  The gemara explains that he derived a kal v'chomer from the pasuk אילת אהבים ויעלת חן that Torah provides protection, and so he was not worried.

R' Elchanan in Koveitz Shiurim asks: but isn't שְׁכִיחַ הֶיזֵּיקָא an exception to the rule of shluchei mitzvah ainan nizokin?!  

You have to say, writes R' Elchanan, that talmud Torah is categorically different and provides a more encompassing degree of protection. 

Based on this, I would say there are 2 distinct dinim: the normal din of shluchei mitzvah, learned from the pasuk in our parsha, and the more comprehensive protection which Torah affords, which is learned from the pasuk  אילת אהבים ויעלת חן.

Rav in Pesachim was speaking specifically about talmud Torah, which is categorically different from other mitzvos.  The gemara in Kesubos shows us the upside to T"T, namely, when one is engaged in learning the protection encompasses even situations where שְׁכִיחַ הֶיזֵּיקָא.  Rav shows us the downside, namely, that there is no real חזירתן, as ideally one should constantly be engaged in talmud Torah, v'hagisa bo yomam va'layla, and one is always on the clock. 

(Among other problems with my thesis: why does the gemara in Pesachim introduce Rav's statement if T"T is different than other mitzvos?)