Why are only some Makos listed, and they are not in order?
Radak: They are written in the Torah (they need not be repeated). It mentions some, to recall the signs. Also Mizmor 78 did so.
Malbim: It begins with darkness, for this was the last Makeh (except for Makas Bechoros). There were three sets of three Makos, to teach about three matters. The last of each set did not have a warning; it was merely a punishment. In Makas Bechoros, Pharaoh did not harden his heart any more, and sent [Bnei Yisrael]. The verses say that Hashem sent Osos and Mofesim until [and including] darkness - "Ani Hichbadti Es Libo Lema'an Shisi Ososai Eleh b'Kirbo" (Shemos 10:1). After this, it begins to list Makos until darkness.
What is "va'Yachshich"?
Radak: It made the land dark.
Why does it say "v'Lo Maru Es Devaro"?
Rashi: The Makos that He commanded about them came via His command, and did not deviate from His word.
Radak: Moshe and Aharon did not rebel against His word. Even though Pharaoh scowled at them, they went.
Radak citing a Midrash: The Beis Din above did not rebel against His word.
