Difficult to say... depends upon how you read it.
For 2 and 3, I would never have the Skill version be better than the Talent (except being resistant to magic draining effects), and unless the description specifically mentions a difference (as some do), I would have the effect be identical. Also, unless there are specific rules stating it, I would not say that any talent works differently for certain disciplines (such as Windmasters) than anybody else.
In any case, a windling flying under their own power is supporting their own weight. I would say the phrase about "stands their ground" has no bearing upon flight. So under all circumstances I would say a flying windling can use Wound Balance. Because they are clearly supporting their own weight as opposed to being carried.
As for the Flight Spell. Like I said, it depends upon how you read it.
when making any Knockdown tests while supporting and balancing his own weight (as opposed to being mounted or carried by another character)
It depends upon whether you consider that the spell "grants the power of flight" (as the spell description says) or whether you consider that a flight spell is "supporting" them. It also depends upon whether you put the most emphasis on "while supporting his own weight" or on "as opposed to being carried". On the one hand, he is not being supported by a mount or another character. So if you interpret "while supporting his own weight" as being simply "not being carried by anybody else", and the flight spell grants you an ability to fly, then you could say that you are flying and your own flying is supporting your own weight. Thus you could then say 'Yes', Wound balance could be used.
But if you interpret it that you are being supported by the flight spell, then 'No' you can't use wound balance.
Note also that this distinction affects other things. I have heard wizard players argue that a character with flight cast upon him does not fall down (and suffer falling damage) when knocked down. They argue that they just start to hover upside down or sideways at the same height they were before being knocked down, because the flight spell is supporting them. If the GM accepts this (dubious in my opinion) argument, then no character with a 'flight' spell could use Wound Balance, since the 'flight' spell is indeed supporting them. But if the ruling is that the 'flight' spell simply gives an ability to fly, and that that ability can be temporarily interrupted by being knocked down, then it might follow that the characters ability to fly is supporting their own weight. Thus Wound Balance is appropriate.
My own logic chain would be that windlings flying are supported by their own weight and can use Wound Balance while flying. The Metal Wings spell gives a character wings, the wings give a character an ability to fly and the wings support the subjects weight, so recipients of that spell can use 'Wound Balance". The flight spell uses almost the exact same words about gaining an ability to fly and having a flying movement rate, so yes the recipients of that spell can also use Wound Balance.
But it is all subject to GM interpretation other interpretations seem valid.