I don't have an N93, but you're not getting any love from those who do. I'll try to point you in the right direction.
1. I have a feeling if you searched you'd find that answer. Play with the outer display settings, check the theme settings, and check the time and date settings. It's probably in one of the three.
2. You'll have to search for that one too. Natively, I know it plays MP4, but I don't know about any other formats.
3. The videos should have a film reel bar-looking thing on the left side of them to differentiate already. As a workaround, you could use Y-browser to change the video files' attributes to Hidden so they don't show up in the gallery, but then you'd have to use Y-browser or another file explorer to access them.