1) Yes. You will suffer the fatigue as damage.

2) The important point is that the monster is within 2 spaces. In other words, the range from the attacking monster to you has to be 0, 1, or 2. There is a significant difference between a figure and an obstacle- namely, you cannot count through obstacles (obstacles affect range). See my range and LOS guide for more info.

3) There is no restriction against trading search cards which have already been flipped. Remember that some search cards are discarded, not flipped.

Thx, So for situation b (B= figure), need I consider LOS to counter attack ?

Well the monster itself has to consider LOS in order to attack you in the first place. In situation a, the hero can see the monster (and vice versa), in situation b LOS is blocked by the obstacle.

If B would be a figure instead of an obstacle, LOS is still blocked and the monster cannot attack unless a skill allows it to (However, now you can count through the figure which may be relevant for skills).

If the monster can attack, I am the Law can be used. Similar, if the monster would attack another hero that stands next to the marshal and the marshal is affected by the attack via (say) blast, then the marshal can use I am the Law if that monster is within 2 spaces of the marshal. LOS is not required for this skill, but counting spaces.