Moed Katan perek 3 - aveilus and the sources for it

In the third perek (and the end of the second) is the main discussion in Shas of the laws of aveilus. Now there is apparently some agreement that most of the laws of aveilus are m'd'rabanan, with a few details from the Torah. I'm guessing that is because most of the pesukim brought to derive the laws are from Tanach, not Chumash.

I've made an impromptu list of the places in Chumash and Tanach that are used as sources for these laws. Here's the list:
Chumash - the hesped for Yaakov Avinu, the (lack of) mourning for Nadav and Avihu, the laws of when cohanim may be tamei for their relatives and when not, the requirement to bury someone who received skilah, the confession that "I did not eat (maaser sheini) when I was an onen". From Nach - Iyov's mourning over his children, Yechezkel's (lack of) mourning, David's mourning for his son, Yoav's arranging a woman to pretend to mourn in front of David. [Mephivoshes also mourned when David left Jerusalem, but I don't recall if anything was learned from there.]

But here is the thing that amazes me: Every one of these sources is indirect. The Torah or Tanach refers to something that someone was doing, never commands anyone to do it. Even the ones that are in command form, like the cohanim, are basically describing when cohanim may or may not do like everyone else is doing already. Same for the niskal.

Why does the Torah never command aveilus, whether d'oraysa in Chumash or d'rabanan from Nach? Derech eretz kadmah latorah?
What other mitzvos are similar?

Update: by now I'm at the end of the masechta, and I have encountered a bunch more sources, but I don't think I've seen exceptions to this note.

Updated complete list [for gemara Moed Katan; I still need to check Maseches Semachos]:

Chumash -
The seven-day hesped for Yaakov Avinu (the Rambam says that unlike the dinei Torah, this one is only d’rabanan and the verse is an asmachta)
The (lack of) mourning for Nadav and Avihu (haircuts, tearing clothing, crying, laws of aninus and kodshim, one day of aninus)
The laws of when cohanim may be tamei for their relatives and when not (obligation to mourn and for whom)
The requirement to bury someone who received skilah the same day (obligation of kevurah)
Viduy maaser (aninus and eating kodshim).
V’samachta b’chagecha (issur to mourn on Yom Tov)
Miriam’s burial (immediate burial for women)
“The days of crying for Moshe” (how many days not to wear tefillin)

Nach -
Iyov's mourning over his children (sitting on the ground, tearing his clothes, tearing when standing, comforters remaining silent), mourning sits at the head
Yechezkel's (lack of) mourning (not putting on tefillin, not learning Torah or speaking too much, hesped, crying, wearing shoes, wrapping one’s face, eating seudas havra’ah.
David mourning for Shaul (tearing clothing a tefach, hesped, crying, fasting for talmid chacham, tearing for av beis din or nasi, tearing for disaster happening to the nation)
David's mourning for his son (washing, annointing, relations)
David mourning when Avshalom killed his sons (tearing clothing standing, sitting on ground
Yoav's arranging a woman to pretend to mourn in front of David (not laundering, not annointing or washing.
Mephivoshes also mourned when David left Jerusalem (cutting nails and mustache, laundering clothing)
Amos “I have turned your festivals to mourning” (prohibition of work, aveilus 7 days); “a bitter day” (1 day of aninus)
Elisha tearing for Eliyahu (tearing for one’s parent or rebbi, tearing across the edge)
Yirmiyahu Destruction of Megillas Eicha and they didn’t tear (tearing for sifrei kodesh)
Yirmiyahu (don’t mourn or cry too much, crying for those who die without children, different types of crying/nehi/kinnah)
Yirmiyahu (burning for kings)
Koheles (funeral has precedent over wedding)
Chizkiyahu hearing giduf (tearing for giduf)
Yishayah: People arrived from destroyed cities and Tzion (kriyah)
Yishayah: in hesped strike on the heart
Yoel: tear your hearts not your clothing (kriyah till the heart)