Regarding tnai on mitzvos that we touched on yesterday (link), Anonymous said a nice sevara in the name of R’ Tzvi Pesach Frank that many other Achronim echo as well. How is giving a ring for kiddushin on condition different than trying to chop off a chicken’s head and stipulating that you want it to remain alive – the outcome of the act is inevitable; it’s not under your control to determine whether it occurs or not? How does tnai work? The answer is that the ability to appoint a shliach to create kiddushin proves that the result, the kiddushin, is not inevitably tied to the chassan’s act of giving a ring, but stands on its own, and can be effected by stipulations and conditions.
The test of whether an act can be done through an agent is necessary only in cases like kiddushin, where the goal is to produce some effect, some chalos, and the question is whether that chalos can have conditions attached to it. Where the act being done stands as an end in its own right, obviously conditions can be set -- you don’t need the whole chiddush of parshas t’naim to allow that. Therefore, even though kri'as shema cannot be done by a shliach, a tnai can be made: if one makes the zman later, that later reading should count as the kiyum mitzvah; if not, the earlier reading should count.
(If this logic is correct, that all the rules of mishpetei ha’tenaim do not apply to tnaim on mitzvos, e.g. one does not need tnai kaful. I have seen quoted that both R’ Yisrael Salanter and the Chazon Ish did require tnai kaful even in these cases.)
A side note that I was thinking of when I went over last week's parsha, and found that the Kesav vhaKabbalah says it: when we read "V'hayu hadevarim ha'eileh..." and "V'shinantam l'vanecha" in shema, these are not meant as stand alone mitzvos of Torah study, but are in context a continuation and amplification of the opening pasuk, "V'ahavta eis Hashem Elokecha..." Someone who is truly filled with ahavas Hashem cannot help but communicate that love to others. The way to do so is by learning Torah, teaching Torah, speaking Torah.