Why did Yakov see fit to enquire after Lavan at this point?
Seforno: Because it would not have been proper to turn up without knowing his host's situation.
Malbim (4): He wanted to ask before rebuking them. 1
Perhaps he feared lest they resent his rebuke, and not want to answer him. (PF)
Why did Lavan send Rachel to look after the sheep and not his sons?
Rashi (30:27) 1 : Because he did not yet have sons. 2 Otherwise, he would never send his daughter with his flock, amidst the shepherds. 3 And the sons that are mentioned at the end of the Parshah were due to the B'rachah that came with Yakov's arrival.
Riva (Bereishis 30:27), citing R. Elyakim: Perhaps Lavan had infant sons who were too small to shepherd the flock! If so, he would leave her at home to take care of them. (Lavan's wife had died - refer to 29:6:151:3. Why couldn't Leah take care of the babies, especially if there was only one! - PF) Seforno (9) explains that she was knowledgeable in the art of shepherding.
Yakov asked "ha'Shalom Lo." Why did they answer only "Shalom"?
Hadar Zekenim, from Bereishis Rabah 70:11: In all their answers, they were more concise than Yakov. Therefore they told him "Rachel is coming. She is wordy, like you. Ask her all you want!"
Moshav Zekenim: It is because "Ein Shalom la'Resha'im." He asked if Lavan is peaceful or quarrelsome. To avoid answering, they said "Shalom" (there is peace in the world), and said that Rachel is coming, to divert him. 1
Tosfos ha'Shalem (3): "V'Yadata Ki Shalom Ohalecha" refers to a wife. Lavan's wife had died.
Ohr ha'Chayim: He asked (a) is he Shalem in his body (healthy) and financially; (b) are you at peace with him? They answered Shalom
Tosfos ha'Shalem (5), Malbim: Saying "Rachel is coming with the flock" answered this. If he clashed with people of his city, he would not send her alone! Ohr ha'Chayim
Why did they say that his daughter is coming?
Refer to 29:6:151:1,2,4 and the notes there.
Ohr ha'Chayim #1: They said that she is coming with the flock to show that he is not wealthy enough to hire a shepherd, and one girl can oversee his entire flock.
Ohr ha'Chayim #2: They said that she is coming with the flock to show that he is too stingy (to hire a shepherd) and disgraces himself through letting his daughter graze his flock.
Ha'Emek Davar: They realized that he needed to know about him more than they know, so they said that Rachel is coming; he can ask her.