For questions 1, 3 and 4, Happyjew is entirely correct; only if the Harpy (not including its base or flight stand) is more than 25% obscured is it granted a cover save.
For question 2, models in ruins are granted a cover save regardless of whether they are at least 25% obscured, though it may be difficult for a Harpy to be
in the ruins.
Happyjew wrote:2. Only if the ruin is also area terrain. Otherwise, see number 1.
What's an area terrain?
(I know what you mean, but area terrain no longer exists, and ruins appear to be one of the few instances (the only?) of what used to be described as area terrain)