There is a fundamental machlokes between Rashi and Tosafos as to the source of the exemption of mitzvos for an o'nen. Rashi says that it is similar to a chosson who is mentally focused on doing the mitzvah of p'ru u'rvu, therefore we don't impose on him any other positive obligations. Similarly, the focus on burying one's dead forces chazal to waive the imposing of any other mitzvos. Based on this the exemption is essentially based on עוסק במצוה פטור מן המצוה. Tosafos cites the Yerushalmi that says that the rationale is either for kavod of the deceased, or to ensure that there is someone tending to the burial needs. The Yerushalmi says explicitly that the kavod ha'meis concern would apply even if there are other to tend to the needs of burial.
Rashi and Tosafos seem to also argue whether the exemption of an o'nen from the making of brachos is an exemption, or even a prohibition for the o'nen to make brachos. Tosafos who holds that the rationale is to show kavod for the meis would certainly hold that it would be prohibited to make blessings since that would display a lack of undivided attention to the meis. whereas Rashi who holds that it is an exemption from doing other mitzvos, it would still be permitted to choose to do them.
The gemara asks why does the mishna stipulate מי שמתו מוטל לפניו which implies that the exemption from mitzvos is only when one in in the present of their deceased relative, but the Braisa exempts one from mitzvos even if not in the presence of their deceased relative. The gemara has two possible answers. The first is that the exemption from mitzvos only applies within the presence of the deceased. That approach certainly seems to assume that the rationale to exempt is kavod ha'meis and therefore only applies in the presence of the deceased. But the second approach which is that whenever it is your obligation to bury this relative, it is as if he is lying in your presence, wouldn't compel this rationale.
The gemara also equates the exemption of an o'nen to one who is watching a dead body and is exempt from mitzvos. That would certainly imply like Rashi that the rationale to exempt is osek b'mitzvah patur min hamitzvah, because the two rationales of the yerushalmi would seemingly not apply. Yet in the next gemara it compares an o'nen to one who is walking through a cemetery and cannot perform mitzvos because of לועג לרש - it is degrading to he deceased by teasing them that they no longer have the ability to perform mitzvos. This would indicate that the p'tur of an o'nen is also connected with kavod ha'meis as Tosafos cites from the Yerushalmi.