If a person is uncertain if he will make sof zman kri’as shema if he davens along with the tzibur, simply reading shema early is not the ideal solution. That might guarantee making the zman, but it definitely forfeits the kiyum mitzvah of reading shema in the context of its brachos. A better solution (see R’ Akiva Eiger O.C. 46) would be to read shema early with a tnai that if one makes the zman, the earlier recitation counts only as talmud torah; if not, the earlier recitation counts as the mitzvah of kri’ah.
The problem with this solution is that the reading of shema does not seem to be something one can set stipulations on. The gemara (Kesubos 74a) tells us that a tnai can only be made when an action can be done by a shliach. Tosfos explains that the ability to designate an agent to act on one’s behalf demonstrates control over a process. It is that same level of control which affords the possibility of making conditions. Since one cannot appoint a shliach to read kri’as shema on one’s behalf, one should be precluded from setting any conditions on the mitzvah.
How then can R’ Akiva Eiger suggest tnai as a possible solution in this case? I have seen two basic approaches to resolve the quesion. I’ll leave it for you to mull over for now.