Nope...and if you've seen Breaking Dawn...there is a scene where Leah tells Jacob she wishes she could imprint on someone so that she could break the connection with Sam.....poor Leah....
No, they were just in love. Leah was not a wolf yet and so did not have the ability to imprint yet. Besides, it is possible to just be in love with someone without imprinting.
No. Sam imprinted on Emily, that's why Jacob said to Bella in New Moon, "Welcome to the Leah, Sam, Emily fest." Sam did like Leah, but could not commit to her, for he had already imprinted on Emily.