1 They're new airbenders who found it difficult to do even during their teaching shown in an earlier episode.
2 She's on an airship that keeps repositioning itself around the temple.
3 Is bent lightning magic as well? If so, then yes.
4 The same reason the Professors at Hogwarts protect their students.
5 This sort of intentionally left vague. Previously, lava bending was shown to only be used by Avatars who had mastered the elements. (that one fire avatar with the volcanoes, Kyoshi, and Roku). We have to assume as of right now that it is one or the other because of the amount of effort he put into trying to metalbend.
6 So we could have a cool fight scene with the Beifongs kicking butt.
7 Mako has been shown to control lightning before, tho I dont remember a part with flight. Both of these are sort of implied to be much more common as time has moved on. (Workers using lightning in the factory, General Iroh using it to maneuver between planes)
8 They'd be giving their enemy a group of people who know where their base of operations (of sorts) is right now.
9 They dont, though it probably seemed smart to isolate Korra before attempting to kill her (in the Avatar state no less)
"In the northern air temple fight, for example, we saw the combustion woman on the airship and she fires a beam at the stable. We see Tenzin struggling in a fight with a less experienced airbender. It should not be a hard fight for him. "
I'm not sure if Tenzin has had much experience in fighting airbenders due to them being wiped out until very recently.
"Then, we see the airship move a bit and combustion woman fires at Tenzin. As the show has been VERY bad about establishing where people are and when, it is a surprise. But, it is a cheap surprise, a contrivance. There is no reason to think the awkward and slow maneuvering airship can have a view of all of the temple."
P'Li is established to have been dealing with Kya and Bumi. After they are taken care of, she moves in on Tenzin. Tenzin is also outnumbered trying to fight off Ming-Hua, Ghazan, and Zaheer.
"WHY doesn't Tenzin run?"
He is still attempting to fight them off to protect his students. He states he won't stop fighting while he's still breathing. (Which is a hugely ironic choice of words due to the previous episode's death of Earth Queen. Gave me an oh crap moment on the chance Zaheer decided to kill him.)
