Ask your DM
We have been playing it as described in 1., stuck in the webs, which has not caused any issues.
Strict RAW, spells only do what they say they do. The spell does not say the creature is fixed in place, and neither does the contrained condition.
The webs count as difficult terrain, which matches the idea that the creature is not in air any more, but stuck in a wad of webs. That however is also is not explicitly stated. Difficult terrain’s rule effect is merely that movement speeds are halved. You could interpret this as the webs being too thin and the creature still falling, maybe at half speed (I’m not sure if passive falling counts as movement).
So the strict RAW seems to indicate the creature would fall. By the core rules, you fall straight to the ground, as in 2. Xanathar has alternate rules which have you fall just 500 feet, and with a reaction from a readied action you might be able to resume flying when you use those rules as you leave the web. These questions have already been answered separately.