I don't use Macs, so I've been hesistant to give you specific answers, but I'll give it a shot.
1. It should be ok to use QuickTime film. On PC's, I'd choose AVI, but since QuickTime is the native Mac format, that should work fine for you.
2. I don't know what the Strobe-like effect is, but if you want your project to be 24fps, then choose 24fps.
3. I don't see a question here, so I'll skip it.

4. As long as you create a 16:9 project in AE and interpret the footage as 16:9, you should be ok when you render your final film from AE.
5. I don't know iDVD, but if the DVD is encoded in widescreen mode, then, yes, the DVD player will expand and letterbox it when you view it on a TV.
6. If you're talking about putting still images into your AE project, then it's best if they are also DV-PAL Widescreen because there will be no resizing/resampling. If your image program doesn't support that (it sounds like yours does, though), you can use square pixels and "Interpret Footage..." in AE.
7. If you're talking about the field order in interlaced video, DV video is always lower field first (PAL and NTSC are the same in this regard).
Justin