Hi.

1. I don't think there is actually some SDK that you can download but maybe I'm wrong. The PDF you are reffering to, has all the info you need to configure CS Weapon for third party planes. With this I mean, how you can fit the CS 3D Colimated HUD to third party aircraft.
It's a lot of work (needs many - many tests until accomplished) but If you care enough for some particular plane, it may worth the effort.

Good news for you are, that Razbam's A-7 HUD is already "made" from another user & you may download it from CS Liveries (Site, not forum). There you may find many interesting things also, like weapons, missions, etc.

2. You cannot attach weapons to any aircraft. Weapons will only show up at the moment they are triggered. For planes (like A-7) that have external weapon loadouts (by the plane developer I mean), you can use this for the "effect" & immersion.

3. Since fitting the colimated HUD in a 3d party plane, is really a lot of work, I preffer to adjust the aimpoint at plane's croshair point (if there is one of course). At the same time, this is much more realistic & immersive than having the F-14 or F-4 for example with an F-15 HUD (the CS 3D HUD is "F-15ish") and much too easier (needs very few tests to accomplish the right spot).

http://www.youtube.com/watch?v=h4DPxgslNxI

Finally, you always have the option to turn on/off the 2D HUD with your screen centered (& without moving your view of course, concentrating on target...)