The M92's videos are old. There is some new improvements and features, look at
Firmware updates, dictionaries, fonts and third-party apps for Onyx Boox M92. The device it's really better.
1/ I've tried with my 6", but it's not possible. A big screen is necessary. Only some EPUB files are usable, but the pictures are a problem : too small. With the M92, you can annotate the texts.
2/ With the M92, there is no problem. You can define the zoom level, you can read in landscape mode too, read multicolumn documents, etc.
3/ I'm not sure to understand your question. The M92 is seen like an USB key and can play MP3 files.
4/ On the M92 you have a filesystem. You makes your folders and put your files in. When you have many documents, it's more interesting. You can use an SD card.