Text and tradition: their place in "acquisition of the Torah"

The huge baraita in the sixth chapter of Pirkei Avot (6:6) lists no fewer than 48 elements that, in aggregate, are said to represent kinyan HaTorah ("acquisition of the Torah"). The list is an impressive one and, if we are honest, most of us are doing pretty well if we even manage half of them in today's world. Still, they represent an ideal -- and a challenge for us too, even if it is quite daunting.

Some of the items on the list are a little puzzling, since they seem obvious. Two that stand out in particular are Mikra (the text of the Torah and indeed the prophets and writings that comprise the rest of the Tanach) and Mishnah (the tractates that comprise the Six Orders of Mishnah and their accompanying Talmud). Why single these items out for special mention in this list? After all, they together add up to the content of our Torah learning -- and it is not possible to learn Torah without learning Torah (there is a small exception in Eruvin 100b, that one can learn Torah from animals such as cats, ants and cockerels, but the amount that can be learned from them is strictly limited). 

Perhaps the intention of the author of the baraita, when including Mikra and Mishnah, is to stress that it is only the text of Mikra and only the tradition of Mishnah that lead to acquisition of Torah learning.  They are listed in our baraita only to exclude writings that are not part of the canon of Tanach (works of apocrypha and pseudepigrapha and those which fell out of favour such as Ben Sira/Ecclesiasticus) as well as extraneous writings such as ancient Jewish fiction.

Exclusion of anything that falls outside the scope of the Tanach and the Six Orders of the Mishnah and their derivatives still leaves one question open: how does one categorise midrashic literature? While this falls outside the narrow interpretation of Mikra and Mishnah, in general it provides explanations and discussions based upon them. The author of the baraita, being a Tanna, would have been familiar with Midrash and may even have authored midrashim himself so I would like to assume, in the absence of evidence to the contrary, that the term Mishnah embraces Midrash too.