What did Vehigbalta es ha'Har" entail?
Rashi and Rashbam: Moshe had to actually place markers along the borders of Har Sinai and to warn Yisrael not to cross them.
What are the connotations of "Leimor"?
Rashi: It means that the markers would 'tell them' not to go past them. 1
Otherwise, it would have been more appropriate to write "ve'Amarta aleihem". See also Sifsei Chachamim.
Why were the people forbidden to ascend the Mountain?
Oznayim la'Torah: Because Moshe was having a private audience with Hashem, nobody was allowed to be present, 1 (even though they were permitted to listen).
Why was a seemingly- minor infraction (passing the boundary) punishable by death?
Seforno: To serve as a deterrent
How will someone who transgresses die?
Moshav Zekenim: People may not kill him