I really don't see how opera mini would or could make a difference.
For what it's worth, I had a mp4 video podcast that wouldn't play. Actually, I got the audio track, but black screen for picture. Experimenting, I used media manager or whatever it is in desktop manager to 'convert' the video for blackberry, and it somehow fixed the mp4. It was still an mp4, but it played. It really wasn't worth the time because it took as long to convert as to just watch it on the netbook. But there you go.
I think your issue is something with the original file, and not your browser. One thing you could try is upgrade your OS to the latest version for your model, if that's an option for you.