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.
Why did Lavan send Rachel to look after the sheep and not his sons?
Rashi (later on 30:27): Because he did not yet have sons. 1 Otherwise, he would never have sent his daughter 2 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.
It is unclear however, how Chazal knew that Lavan did not have young sons who were too small to shepherd the flock. As a matter of fact, the Ramban later on (See 30:30), suggests that this was the case. See also Seforno on Pasuk 9, who, commenting on "because she was a shepherdess", explains that she was knowledgeable in the art of shepherding.
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. 1 To avoid answering, they said "Shalom" (there is peace in the world), and said that Rachel is coming, to divert him.
Tosfos ha'Shalem (3): "V'Yadata Ki Shalom Ohalecha" refers to a wife. Lavan's wife had died.
Tosfos ha'Shalem (5): Saying "Rachel is coming with the flock" answered this. If he clashed with people of his city, he would not send her alone!