Question: A surface read of Pirkei Avot and Hilchot De'ot would suggest that their contents appears to be primarily advice as to how one *should* proceed over and above the minimal halachik requirements. Nonetheless, these two respective works are codified among primarily halachik series, the Mishna and Mishna Torah respectively. 

Do the instructions provided in these two volumes (and other volumes that fit my aforementioned criteria) contain halachik weight for anyone or everyone? Are they codes to teach us how to behave should we wish to go beyond the letter of the law? Are they simply there to enrich our lives?

Answer: 
Rambam Hilchos De'os is halachikally binding except where the Rambam specifically states that it is limited to a Talmid Chacham (e.g. chapter 5). Hilchos De'os contains the specification of no less than 11 Biblical mitzvos including the prohibitions against revenge and lashon hara and these commandments are certainly obligatory.

The portions that deal with treating others properly like greeting others pleasantly (sever panim yafos) are included in the mitzvah of emulating Hashem and are also binding. See 1:5 where this is explicitly stated by Rambam.

Similarly, Pirkei Avos, in general, applies to everyone; note that the Ein Mishpat (found in Vilna Shas end of Maseches Avodah Zara) has numerous footnotes citing the passages in Rambam and Shulachan Aruch that are based on the referenced mishna. In addition, in many Halachic contexts Tosfos on the Gemara (e.g. Rosh Hashana 4a s.v. Beshvil) raises questions from Avos (but also see Tosfos Yom Tov ibid and rambam end of hilchos teshuvah) who clearly maintain that this is not required but is certainly optimal). (Of course, some mishnayos in Avos were intended for Talmidei Chachamim e.g. 1:1 Shimon Hatazdik etc) Some mishnayos are intended as not required advice (). One has to see how the Poskim (e.g. Rambam, Shulchan Aruch, etc) understood those comments to determine if they are binding or simply advice.(e.g. Avos 1:5 "al tarbeh sichhah" and Rambam Hulchos De'os 5:4)

Your question is similar to the one raised by the Chofetz Chaim in his introduction to that sefer where he asks whether Rabenu Yonah's Shaarei Teshuva is halachically binding. Chofetz Chaim clearly understands that Rambam in Hilchos De'os is binding as he quotes from it extensively throughout his sefer. As a matter of fact, the issur for lashon hara is found in Hilchos De'os!
the torah is written in a universal form so that every Jew is able to fulfill it.

Nevertheless, G-d's intent is that those who can do more should strive to. This is known as the urge of the understanding.

The prescriptions in Pirkei Avot are in this category.

Most people are not able to fulfill all the words of pirkei avot, so it is not to be looked as 100% halachically binding. Nevertheless everyone at his level of understanding must try to fulfill whatever he can.

This is explained in the Chovos Halevavos, Gate 3 ch.3:

> 
> Third, the intellectual urge cannot include equally all who are under
>  the obligation of service, because some human beings are of limited
>  intelligence, while some are superior in understanding. But the urge
>  of the Torah applies equally to all who have reached the status
>  subjecting them to this service, even though they vary in their
>  understanding of it, as we have noted at the close of the first
>  treatise of this book.
> 
> 
> It sometimes also happens that an individual falls short in some
>  duties and exceeds in others. The intellectual stimulus varies in
>  different individuals in accordance with their capacity of
>  recognition. But the urge of the Torah is not subject to variation.
>  Its form is the same for the child, the youth, one advanced in years
>  and the old man, the wise and the foolish, even though the resulting
>  practice varies in different classes of individuals. And so Scripture
>  says in regard to the all encompassing character of the instruction of
>  the Torah for all the people, "Gather the people together, men and
>  women, and children, and the stranger that is within your gate, (that
>  they may hear and that they may learn and fear the L-ord your G-d . .
>  .)" (Devarim 31:12). Further, it is said, ". . . you shall read this
>  law before all Israel in their hearing." (Devarim 31:11).
> 
> 
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