Its not that slow... just look at what it did to the battleships. It fired multiple times and each one lasted a bit and kuvira could move the blast while firing. I think the main issue with airbenders is that they are not technically flying. Its a far cry from what zaheer did. They are not as fast and changing directions takes a tad longer. Even the bison had trouble there. And the blast had a wide range. With korra it would be different. She can fly using airbending and firebending so she would be more mobile and harder to hit. They should simply go use earthbending to get under it and bury it though.
Slow enough. I mean, it is really hard to hit something that's flying. Hitting a battleship is one thing, or a flying bison(bison's not famous for speed.), but anything faster is going to be nearly unhittable; because you just can't lead something with that cannon. And, since it's mounted on the right arm/shoulder, that means anything on the left side going to be safe until the entire thing pivots.
I am not sure it would be so weak as a front line weapon. As far as we have seen to begin with that thing is by no means slow. It is basically the armored giant. It is presumably immune to metalbending and it can probably withstand damage from rocks, water, earth, air and fire just as well as the normal suits. The thing can basically step on anything. And it is not even slow moving, it walks at a considerable pace and it might even be able to run. Sure, the laser is the most dangerous aspect of it right now because it is such a powerful ranged weapon which barely takes time to recharge and fires at a considerable rate and can obliterate anything it fires upon but even if you take that away it would simply walk onto anything it wants to stomp. So the issue with the mech is that at a range it can fire the blast, in melee it can do serious damage via punching, stomping and whatnot which is serious because no matter how you look at it that thing is neither slow nor clumpsy (in fact it is precise and fast enough that kuvira can afford to control it via metalbending weird controllers). Once you get past that you are basically on it and you need to get inside. Realistically speaking the only spot at which you could get inside (because that thing is likely immune to most forms of bending) is the windows from which kuvira looks out.
The cables would not help here IMO. Assuming they get that close to it they are well within range to be stepped on. And they would have to physically restrain for it to trip. In the series physical strength exceeds that of real life but in here we are talking about the guys holding back something which weights thousands of tons and has the strength to move said thousands of tons. If we go purely star wars style each metalbender would need to have enough metal to spin around the legs of the thing several times. I don't think they have that much. And even that relies on the cables not snapping. And even if you trip it it is not enough, there is no reason for it to not be able to stand up or use its hands to remove the cables.
Korra doesn't have her past lives anymore right? So she would likely be restricted in Avatar state to more powerful versions of the things she already knows how to do. So, since I don't think she can lightening bend normally, she probably can't in Avatar state either.
True enough, she is probably limited to the stuff she knows. Still, that leaves a lot of questions. Why didn't she do it when she had 200 or so avatars backing her up? Aang not doing it is understandable but korra should not have the same moral boundaries. There is also the question of why metalbending has not turned up in her avatar state. Korra is supposed to be brash, headstrong, impulsive and whatnot and yet she has shown almost absurd restrain when it comes to her avatar state. The entire second season should have been avoided if she had simply used her avatar state like she is supposed to.
No idea.
