How did Par'oh know Moshe and Aharon's names?
Ramban: He asked them their names, and addressed with respect.
Whom was Par'oh referring to when said ''L'chu le'Sivloseichem"?
Rashi, Ramban #1 and Rashbam: He meant that they should return home to indulge in their personal work. 1 Shevet Levi was not enslaved.
Ramban #2: Since they came to him accompanied by all the people, Par'oh simply rejected their request and dismissed them all, ordering the people to go back to the heavy work 2 that they were doing for him. 3
Ramban #3 citing the Ibn Ezra): Par'oh was telling Moshe, who represented the people) to order them to go back to work.
Rashbam: He meant that they should neither be idle from their personal work nor should they cause others to be idle from his (Par'oh's) work. Refer to 5:4:3:1, and see also Ramban DH 've'Rashi Piresh'.
Oznayim la'Torah: Interestingly, Par'oh himself described their work as 'Sivlos', and not just Avodah.
Later Moshe and Aharon returned alone (See Va'eira, Sh'mos 7:10), and Par'oh treated them with more respect. See Ramban.
QUESTIONS ON RASHI
Rashi writes that Shevet Levi was not enslaved. What is the reason?
Ramban: It was customary for every nation to designate their sages to teach the people their laws and customs. Consequently, Par'oh exempted the tribe of Levi (by Divine design) from the slavery.