1. When a file is about to be saved, the file name, size, etc., is put in the directory, then the file is saved. If the file never gets saved, it's still in the directory, so you see it listed, but it has no size.
2. "Can't open" a file usually means that you don't have an app that can open that file type. (Say you download an .ogg file.) But since your files aren't saving, I'd say that the difference is in whether any space has been allocated to the file or not. If no space has been allocated to it, it's empty. If space has been allocated to it, but it hasn't been saved, it can't be opened. (The beginning of the file has to match it's type in the file name - if hasn't been saved, there's no "beginning".)
It looks as if it's all the same basic problem - some files, that you get in some way, aren't storing properly. It's probably a kernel problem (but you'd have to install a terminal app and know a bit if Linux to find out), but the "fix" is probably just reflashing the firmware. Since it's a Samsung, that's easy - see
[Samsung] How to flash Stock ROM via ODIN. (Make sure to back up anything you don't want to lose - the new firmware doesn't have your files in it.)
