(Answered) LOS falling on the border between hexes

[TurkinaBoi]

When playing AS on hex maps we often have the situation described on page 69 where the LOS goes straight along the hex border. When deciding if there is cover, forest, or if the units see each other, the book says "the
defender determines which hex it passes through" for the whole combat round, but who exactly is the defender in this particular case? Is it always the person who was determined as "Defender" by the scenario, or is it the player who goes second when resolving attacks? Or something else?

[nckestrel]

The target when the attack is being declared and LOS is being checked.

[TurkinaBoi]

But both players/mechs may fall under that definition that turn. So here's two situations, let me know if my thinking is correct.

We have player A with mech A, and player B with mech B. Player A has won initiative. Mech A and mech B stand in a way that their LOS falls perfectly between hexes, one of which is blocking LOS.

Situation 1:

Both mechs shoot at each other. In this case defender is the one that is shot at first - mech A, because player A has won initiative. Player A decides that the LOS is blocked, so neither mech will shoot at each other this turn.

Situation 2:

Mech B goes first because player B has lost initiative. But for whatever reason, Mech B does not shoot at mech A (maybe it has no damage in that range or targets something else). So mech A never was the target/defender this turn. Then player A is resolving attacks. Mech A shoots at mech B. Mech B becomes the defender as defined on page 69, so player B becomes the decision-maker about the LOS.

So I guess the intention in this case is that defender is the first mech that gets targeted this turn on that line between hexes?

[nckestrel]

So I guess the intention in this case is that defender is the first mech that gets targeted this turn on that line between hexes?

Yes. 
We don't assume you go declaring all LOS along hex borders until somebody actually goes to make an attack that involves one.  Then the target of the attack determines which hex it goes through, and that choice is used for the entire combat phase.