1) For both NTSC and PAL what you have listed are the max resolutions for DVD's. Or Full D-1 There are other valid resolutions. It's also the resolution mini-DV record in.
3) D1 is not an aspect, as mentioned it's in reference to the resolution. Note that resolution and aspect are two completely different animals and can be set independent of each other. 720x480 can be either 16:9 or 4:3, these are the only valid aspects for DVD. Sources wider than 16:9 have black bars added to the video file itself to maintain aspect.
As far as how this relates to video on the internet:
1) It can be anything but since your typical source is from a mini-DV cam which is 720x480(PAL576) you should use that or a lower resolution. Scaling video up provides no benefit and generally produces poor results. Instead let the player scale it.
2. Framerates can be changed but doing so incorrectly will produce choppy video. There is a benefit here, each frame requires X amount of bitrate. By reducing the framerate you can reduce the bitrate which results in a smaller file.
3. This is only applicable to video you're supplying a direct link to. Aspects can be set to whatever you want but whether its going to be respected by the player is another story. The best practice is to encode at a resolution that matches the aspect to insure the aspect is played back properly.
4 Most video cams record interlaced, there are some that record progressive. Each frame of video actually contains two frames split into fields. This looks great on TV designed to play it but not so great on a Computer. You can see this during fast motion particularly, it produces a stair-stepping affect. To combat this you can deinterlace the video but this is a destructive process. There are a few methods but the end result is you are throwing half the frame away.