Y'all are over my head with some of the tech talk, but I'll try to answer the questions:

Blackbird & Clueless - Yes, the files play when completely downloaded.

Blackbird - Using "Videolan" I can preview the file and watch it when complete. This does not include using "Rosetta" .
What is really nice about "Videolan" as well is that you can drag a scroll bar in the controls window that will take you right to the last part of the movie that has been downloaded. Also cool, using "Open File" and "Recent" you can jump directly to the movie without a bunch of clicks.

Clueless - Thanks for the "Rosetta Help" paragraph.

Blackbird & Clueless - I have used "Rosetta" while a file is being downloaded. It takes whatever is there and splits it up into .mpg-1, mpg-2 mpg-3, etc. These files open only in "Rosetta". None of the movie apps I have will open them. I don't know if "Rosetta" having translated or fixed them has had any effect on the original file or not. Since "Videolan" is working so well, I haven't needed to use it to accomplish the original task.

Clueless - Your second paragraph perfectly describes what I have seen. Since "Videolan" is working perhaps "Quicktime" isn't as robust at decoding on the fly.