If I take a lock on Seevor and then shoot him - can he trigger his ability to break my lock even though the token is assigned to him? Does the ship taking a lock count as having a blue lock token as in 1e or does the Jam token just sit there?

A Jam token removes green token or breaks one lock the Jammed ship has, which of the two options is chosen by the person applying the token. The token only remains if the Jammed ship has no green tokens or locks.

You lose the lock before you can use it to modify, but after using it to declare a weapon, if you are using one that has a lock requirement.