Here it says "va'Eshev ba'Har." Below (10:10), it says "v'Anochi Amadti ba'Har"!
Rashi (like R. Yochanan, Megilah 21a): "Va'Eshev" means 'I remained'. 1
Rav (Megilah 21a): Moshe learned while standing, and reviewed while sitting.
R. Chanina (Megilah 21a): He did not stand or sit; he leaned. 2
Rava (Megilah 21a): Moshe learned soft (easily understood) matters while standing, and difficult matters while sitting.
Sometimes 'Amidah' means delaying, and not specifically standing, e.g. "v'He'emidah Lifnei Hashem" (Bamidbar 5:16, according to Chizkuni; it need not exclude sitting, for only Malchei Beis David may sit in the Azarah - PF), "v'Amad v'Amar" (Devarim 25:8; some say that the Yavam need not stand during Chalitzah
Why does it say "Luchos ha'Even"?
Da'as Zekenim #1, Rosh (from Eruvin 54a): One who does not make Lechayav (his jaws) like stone, which is not dissolved 1 , to engage in Torah, will not merit Divrei Torah.
Da'as Zekenim #2: Most death penalties in the Torah are via stoning with stones.
Da'as Zekenim #3: They were in the merit of Yakov, about whom it says "Even Yisrael" (Bereishis 49:24).
Da'as Zekenim #4: They were in the merit of the Beis ha'Mikdash, about which it says "Yisad b'Tziyon Even" (Yeshayah 28:16).
Da'as Zekenim #5 (citing R. Shimon): They were in the merit of Melech ha'Mashi'ach, about whom it says "Isgezeres Even" (Daniel 2:45).
Rashi (54a): I.e. even if many trample on stone, it is not dissoloved. So his jaws do not weary from reviewing his learning and teaching others.
Why is "Luchos" written Chaser (without a Vov)?
Da'as Zekenim (8:7), Rosh (8:7): (We can read this Luchas.) They were identical, like one. 1 There were two, corresponding to Chasan and Kalah, two Shushbinin (close friends of the Chasan), Heaven and earth, written and oral Torah, this world and the world to come.
This is unlike Da'as Zekenim wrote below (10:1), that five Dibros were on each! (PF)