From: The Great White North!
I think Ancalagon answered #3 quite well so I'll tackle #1 & #2. I have not played this scenario, or if I did I cannot remember it, but a few thoughts:
1) I attack the airfields sequentially, but should I aim to attack all airfields at the same time?
This depends on a lot of things but when in doubt revert to the principle of war: Concentration of effort. Shutting down an airfield effectively is what you want to do, if you disperse your efforts over several airfields you may only damage them and that means the bad guys can still use them. So its in the scenario name: Hit hard. Then you move on and hit something else hard, sequentially unless you have huge resources. The obvious question therefore is - what do I hit first? You need to use any information available to you - the briefing, recon, Wiki etc to decide this. Set your priorities, air superiority is usually the highest so find his fighters and shut them down, this will make the rest of the campaign much easier.
2) When attacking the airfields closest to Cairo and the Nile, do you use a minimum number of aircraft in each wave? To me it seems like using 3 or less aircraft, then there is a high risk of aircraft being shot down
I'm an artilleryman, and we have a saying: Go big or go home! If your aim is to shut down a base, then do that. Its no good 'almost' shutting it down. You'll be tempted to be efficient in your allocation, OK that's nice but accountants don't win battles - don't be wasteful, especial when you have limited weapons, but don't be stingy either.
Hope that helps.