I don't understand the problem.
First, a character is not flat-footed outside of combat. Flat-footed is defined in terms of being in combat, i.e., in initiative. No combat, no initiative, no flat-footed. It's irrelevant that feather fall is an immediate action, outside of combat, just as it's irrelevant that stone shape is a standard action, outside of combat. Outside of combat, feather fall is just a spell you cast to slow a fall.
Second, if you are in combat and flat-footed, I don't understand why it's so unthinkable that someone deliberately trying to make a wizard's teammate fall might actually succeed in doing so. Why is it so difficult to believe that a wizard could be caught gawking by the sudden eruption of combat?
Feather fall works perfectly well as an immediate action spell. (Nerveskitter didn't, but as someone pointed out, nerveskitter was errataed. Badly -- it should have simply said, "when rolling for initiative" -- but well enough to serve.)