1)
Why did Lavan say "va'Tignov Osi"?
1.
Bechor Shor #1: Our city agreed that no one may leave without Os Sheli (my Siman). I would have given it to you! One who leaves without it is assumed to be a thief.
2.
Bechor Shor #2: Lavan's money was dear to him like himself. It seemed to him as if Yakov stole Lavan himself.
2)
Why is b'Shirim written lacking a Yud?
1.
Tosfos ha'Shalem (3): We read it like Sarim (I would have sent nobles to accompany you).