What did "ve'Higbalta 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 does "Leimor" mean?
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 was a seemingly- minor infraction (passing the boundary) punishable by death?
Seforno: If the people, who were already keen to come closer, would break through and see more, many of them would die and Hashem's Simchah would be disturbed, when their corpses rendered the location impure and when their relatives would mourn their death, with the result that the Shechinah would not rest with them.
If one transgresses, how will he die?
Moshav Zekenim: People may not kill him