The desperate defender op card lets a squad attack even if pinned or disrupted.

Does this mean that a squad in Op fire that gets pinned (or disrupted) can still retain its Op fire status and shoot back?

I don't believe so. The rules state that if a squad in op fire is pinned or disrupted, it is automatically fatigued. It's the risk you have to take - should I put my unit in op fire or not?

I know that ordinarily a pinned squad would lose Op fire and become fatigued, but these guys are desperate.

Op cards seem to over ride general rules, but how much in this case? How desperate are they?

The card doesn't state any changes to the op fire rules, so unless there is word in the erreta otherwise, we have to assume that the rules in the book still hold.

This would simulate the fact that a unit that prepares for opportunity fire is also exposing itself - in order for your machine gunner to be able to see advancing enemies he has to have his head visable through the window. He is therefore more exposed to enemy suppressive fire than a fresh unit, who presumable are sitting low with their backs to the wall.

I think they can. When a op fire unit is pinned it is fatigued to make clear it could not attack because it's fatigued. But with "desperate defenders" the unit can attack, so it can remain in op fine and does not become fatigued.

I think this card means that a machine guns continues firing at 1/2 until it's destroyed.

The strategic cards alter the game rules. The problem is rules does not make references to strategic cards as exceptions and cards does not have enought text to make clear how they interact with the standard rules they can affect.

I would have to agree with the others. The main rule that a unit in op-fire that becomes pinned/disrupted is automatically fatigued would still hold true even with the desperate defenders card. Fatigued units cannot go again during that action phase unless you play the action card that allows you to remove a fatigue token from the unit. Had the unit not been in op-fire when it was pinned / disrupted then it would be able to do a 1/2 firepower concentrated attack. A disrupted squad, even with desperate defenders active, would still not be allowed to contribute to an assault since an assault is not a concentrated attack action.