q1: A follower, he may be a knight, thief, monk or farmer, he may be a small follower or big follower, a mayor, a wagon or a phantom, is only one follower, though they may have different strength. (The barn, the builder and the pig are no followers, though they could be meeples.)
q2: good question, did you look in the CAR? It's not mentioned there I think ... I presume there will be only one point: If you have a road with a depicted bridge over another road and they are segments of one road the tile willl only score 1 point, too.
q3: The castle scores, when another building in its vicinity is scored later / in a subsequent turn. When the city is completed with the same tile, the castle does not score.
q4: You may place only one tunnel-chip in a turn on any unclaimed tunnel ending (this can also be the road of someone else!).
q5: You have to put a ferry on the lake. Roads ending at the lake without ferry are completed after placing the ferry. Roads with ferry are continuing. If you extend a road with ferry, you may move the ferry opening a completed road again.