I've seen units on air superiority missions engage bombers that were attacking targets within their base city's radius-my guess is that their umbrella extends over the full 21 tiles a city can work, or at least over those tiles that are currently being worked and/or have friendly ground units in them, but I'm not sure how this works when they are stationed at airfields. I've never seen flak, mobile SAMs, naval units, or SAM missile batteries engage aircraft that did not attack the tile they occupied. In order for them to really have a ZOC affecting aircraft, I think the game would have to have a feature that allowed you to plot a course for your air missions (after all, if your aircraft has the range, you could fly around the offending unit's ZOC to arrive at your target unmolested), which seems like it would be annoyingly complicated for game play, and we all know the AI wouldn't be any good at it. I've yet to see a recon mission be intercepted, and I've undertaken enough of them that good luck doesn't seem an adequate explanation, so I believe there is no interception risk here.
I've yet to have a situation where an enemy city/airfield is close enough to mine to have a fighter conducting air superiority in the same area as one of mine, so I'm not sure on this one-certainly merits some experimentation.