What did Moshe mean when he replied "ba'Mesilah Na'aleh"?
Ramban: Initially, Moshe asked permission to travel through the towns via the main roads, avoiding the fields and the vineyards; now he asked to travel on the inter-city highway that led to Eretz Cana'an. Initially, Moshe said that they would not drink water at all; now he said that they if they drink their water, they would pay whatever price Edom asked of them. 1
Seforno: He countered the king's reply 2 by pointing out that his reason would only be applicable if they were to pass through inhabited areas, but not if they would restrict their passage to the highways, in which case there was no reason to invite conflict. 3 "E'evorah!" - Since the reason
What did he mean when he added with "Rak Ein Davar ... "?
Rashi: He was merely reiterating the fact that all they wanted was a passage through Edom's land, and that they would not harm them in any way.
Seforno: Refer to 20:19:1:2.
And what did he mean when he concluded "be'Raglai E'evorah!"
Rashi: Thos was the continuation of his previous statement. 1