How exactly did Ya'akov 'steal the heart' of Lavan?
Targum Onkelos and Targum Yonasan: By simply not informing him of his plan to run away.
Seforno: By pretending not to know that Lavan had accepted his sons' Lashon ha'Ra, and not to notice that his friendly disposition towards him had changed.
Tosfos ha'Shalem (1, citing R. Efrayim): Lavan planned to steal all of Yakov's property. Since Yakov took it, it is as if he stole Lavan's heart.
How could Ya'akov leave his host of twenty years without saying 'goodbye'?
Seforno: He did not do this out of ill-manners, but because he was afraid that Lavan, with the help of the people of the town, would steal from him. 1
Why does the Pasuk refer to Lavan here as "Lavan ha'Arami"?
Ramban (later): To teach us that, despite the fact that Lavan was a master trickster (a sorcerer to boot), Ya'akov succeeded in 'stealing his heart'. 1
Seforno: To explain why Ya'akov stole Lavan's heart (Refer to 31:20:1:1). Because, had Lavan realized that Ya'akov was aware of his feelings, he would have placed obstacles in his path to prevent him from leaving.