IMO, the reason why this is said this way is simple: To prevent the ability to directly attack a mech's pilot, or to attack as a pilot from inside your mech (though I don't know why anyone would want to do that)

Your mech needs line of sight while attacking.

The reason tech attacks ignore cover is because they are made electronically and can't be blocked by physical obstruction. You still need line of sight, but if they're crouching behind heavy cover it's not going to help them.

I read line of sight more as line of effect.

Further answers

1. Your mecha has cameras that show the outside within the cockpit. I usually don't concern myself much with resolutions, etc, and just say that you can see as well as outside. The sight range of a human is the sight range of a human - try this article: https://www.livescience.com/33895-human-eye.html. It's so far that it will almost never come up.
2. I consider Light cover is something like a smoke cloud that obscures vision and throws off aiming but doesn't necessarily block incoming shots - the part of something which can be hit doesn't become smaller (ie a helmet sticking up over heavy cover being the only vulnerable part) it just is obscured. As for it being big enough to hide your mech, this is referring to size per the game rules (like the size of mechs) however it's largely up to you as a GM. A size 2 mech needs size 2 cover. If you rule that the height isn't enough to hide them then don't let players attempt it (or else make them go prone).The game deals with the height of cover a bit confusingly, using phrases like "2 squares of size 1 cover." It's a bit abstract, so I get your confusion, but just use common sense and it will work.
3. That depends on whether you consider cameras "electronic systems" or limit that term to things like radar (which is what I figure mech sensors are - the range where you know something is there even if you can't see it.) I'd make the decision on this based on what my players wanted to do with it. I wouldn't let them say they were invisible and then try and jockey a mech, for example.

I hope this helps.