1) About when do you want Fly to show up in spell lists: low level, high level, or epic?
Around level 7, but spell-wise only for spellcasters that specialize in movement. Non-specialists don't receive flight.
2) When do you want Long term Flight spells and magic items to be suggested to appear (if at all)?
I'm more open to this, since I don't see treasure needing to appear at any particular level. I'm okay with PCs having a huge income at level 1 (if they're nobility, for example), or being mostly broke with no magic items at level 15. I think it's more of a setting issue (though obviously it relates to balance... if it helps balance the game at level 7-12, throw them in at level 7-12).
3) What is your preferred balancing factor for Fly spell (control, duration, speed, maneuverability, action economy)?
Mainly action economy, but with a sprinkling of speed and duration. Make any ranged attack (spell or otherwise) take a full-round action (the hated full-round!) instead of a standard, and make it take at least a move action to stay in the air. That way, you can't do both (fly and make any ranged attack) unless you circumvent the restriction (Haste, special ability, feat, etc.).
4) Should Flight Magic even have a natural drawback?
Slower speed, less duration (obviously), and maybe even requires concentration to maintain.
5) Should maneuverability return?
Yes. Something less in-depth than 3.5. Pathfinder isn't terrible to me, but something simpler wouldn't hurt.
6) How much should the DMG discuss and provide information on dealing with flight (wind charts and effect, falling rules, flight skill checks)?
I'd like wind charts and effects, falling rules, and flight skill checks all in the PHB, so that players can know what to expect in the game without poking around the DMG. So, it should all be provided, but in the PHB.
7) Should the game encourage mundane tactics to combat hostile fliers or should it just encourage DMs to hand out flight or don't use flight?
It should encourage mundane tactics. Nets are fine, as are ranged attacks: make it so that if you're damaged, you need to make a check or fall. Makes fighting in the air a lot riskier.
8) Mundane flight in core (war balloons, gliders, airships, and planes)?
Core as in assumed? No. Core as in the main three books? Yes. It's not usually my cup of tea, but it could lead to some very interesting stuff for some people, and it's not at of the realm of fantasy to have dirigibles or the like.