If you are editing this in Adobe, then you are going to have to "Export" the video out. Now, I don't know if Adobe will output in MPEG form (since I use Vegas), but if it doesn't, then you'll have to output as an AVI and then encode to MPEG with another program. Lastly, you will have to re-master the files to make it ready to put on DVD. Unlike music CDs, you can't just drag-&-drop any file onto the disc and have it play in a DVD player.
Oh, and as to downmixing to 2.0, I ran across this in my bookmarks. Downmix discussion. I hope it helps.