As mentioned, Shluchei Mitzva is about something happening on account of doing the Mitzva. An example is the Gemara in Kedushin, mentioned above, about falling off a ladder that he only climbed onto for the sake of a Mitzva. It is learned from Olei Reggel, where the Pasuk promises that nobody will harm your property while you are gone, where the vulnerability was because of the Mitzva. This is what ???? ???? ?? ??? ??? ?? means.
On the other hand, in Maseches Shabbos Rav Yosef says??? ???? ?? ????? ???? ????.
And, regarding the Tallis, it is not a magic guard, it is a Mitzva. What happened by Dovid Hamelch, Rebbi Chiya and Moshe Rabbeinu is that the Malach Hamaves cannot gain access during the learning. As mentioned above, Davenning is not learning. Additionaly, murder is not about the Malach Hamaves gaining access. Krias Shema is learning and Rebbi Akiva died while reciting it.
Rabbah bar Nachmeini was also Niftar while learning, but that was because he surrendered, the Malach Hamaves wasn’t Sholet.