All answers refer to the slightly revised 2nd printing (the one with the vehicle cards in the back), but should be applicable to the 1st printing as well.
1) Seems like a airplane can't attack a unit within 12" of the edge of the table since it needs to move onto the table 12" before it can attack. Is that right?
Not necessarily. The rules do not specify that you have to move away from the table edge, only that you have to move 12". So, you could place it on the table edge to indicate its proximity. Then, when the plane's card comes up, select an entry point and flight direction. You could also fly in a parallel line to the table edge, or in a very acute angle, so that your base ends up only 4" from the edge, but the plane moved 12" (or more).
Apart from that, any bombs you drop are dropped on enemy units BEHIND you but on your flight path. Remember to roll for scatter, though.
2) In 2nd & subsequent melee rounds, the lower Initiative unit moves into further contact 1st? It would seem that the winner would be allowed to choose the match ups?
The lower Ini unit moves first, giving the winner (who should have more figures by now) the option to react to these moves and "gang up" on target figures he or she considers important.
3) Does Suppression have an affect on Morale? I didn't see one.
Not in that they affect the rolls. The Suppression markes indicate when a unit must test (more markers than figures) and that some figures (equal to the number of markers) are sufficiently distressed to reduce their shooting and melee rolls.
4) What are the + & - values after the ranges on the weapon charts? I'm assuming they're range modifiers, but I didn't see them explained anywhere.
It doesn't specify it expressly, that's right. The general shooting modifier table lists generic modifiers for short, medium and long range, and the weapons tables list the specific modifiers to differenciate more between types of weapons. So yes, they are modifiers.