What is "Ka'asher Tzivcha" here referring to?
Rashi (citing Sanhedrin, 56b) and Ramban (on Pasuk 12): It (too) is referring to Marah, where Hashem first commanded Yisrael about Kibud Av va'Eim. 1
And what are the connotations of the words "u'Lema'an Yitav lach ... "?
Ramban (Shmos 20:12) and Seforno: "Lema'an Ya'arichun Yamecha" refers to Olam ha'Ba (The world which lasts a long time), it means that Hashem will also reward Kibud Av ... with a good life 1 in this world in the land ... . 2
Why did the first Luchos not contain "Lema'an Yitav lach"?