The Tablets were written before Moses went up the mountain! Not just the tablets; more was written: “the teaching and the commandment.” What are those?

Why did Moshe stand with Joshua only to ascend alone?

Moshe did not know how long he would have to be on the mountain, he did not know how the people would behave while he was away, and yet, his only concern was, “Whoever has a grievance should approach,” Aaron and Hur! Moshe did not charge the elders with guiding the people while Moshe was away. He did not send the leaders to go out among the people and help them maintain the great spiritual heights they have achieved through the Revelation. His only concern was for there to be someone for whomever had a grievance. Why the motion have to specify Aaron and Hur; had he not already appointed a system of judges? Did he not trust the elders to settle grievances?

Why does he not speak to the people about how to behave while he would be away?

The opening of the portion explains some of this:

“And these are the ordinances that you shall place before them.” Rashi explains that God insisted that Moshe not be satisfied with teaching the laws two or three times, but that he teach them these laws with reasons and explanations so that they will be like a table set attractively before them. It’s hard to believe that Moshe would spare any effort in teaching the Torah’s laws.  Is it because so many of them address interpersonal relationships, or grievances?