Rabbi Meir is of the opinion that if a father sub-allocates for the provision of his children after his death, under certain conditions, his instructions have to be followed because Mitzvah l'Ka'ayem Divrei ha'Mes.
Rabbi Meir also holds that ha'Masne Al Ma'a she'Katuv ba'Torah, his stipulation is null and void, even regarding monetary matters (above 56b).
According to the Torah, children inherit their father's posessions without any restrictions or conditions.
Since the father's instructions contradict the Torah, they should have no validity, and the children should be provided their full needs.
The two principles of Rabbi Meir seem to be at odds in this case.
(From a further correspondence:)
It seems that I found the answer to my question regarding the seeming contradiction in the Shitos of R' Meir. Since the case deals with minors (Ketanim), although they indeed inherit the entire fortune without any restrictions, Min ha'Torah, they are not empowered to spend any of it. Arrangements such as an Apitropus, were instituted by the Rabonon. Therefore by a de'Rabonon, principles such as Mitzvah l'Ka'ayem Divrei ha'Mes, can take effect.
Please confirm anyways.
Mitzvah l'Kayem Divrei ha'Mes does not oppose Yerushah d'Oraisa, for we consider it as if the father had gifted the assets in question to his children when he was still alive. Masneh Al Mah she'Kasuv ba'Torah only applies when one attempts to create an entity which opposes the Torah; such as a Nazir who may drink wine, or a wife without a Kesuvah. Here, though, one is circumventing the Torah's requirement and not contradicting it.
If one would want his daughter to inherit when he has sons, or would want one son to inherit more than the others, this would be Masneh Al Mah she'Kasuv ba'Torah. If he grants a gift to one child over the others, it is not considered Masneh Al Mah she'Kasuv ba'Torah.