I've answered your question about why there's no cover, above. To answer your other questions:
1. Amazon, for whatever reason, consider that any file you load yourself is a "document", not a "book". That's just the way it is.
2. The jumbled characters are because the Mobi and ePub versions of books on archive.org are created by a raw OCR process from the page scans. Sometimes the result of this OCR is usable; other times it's just a mess. Raw OCR will almost always require manual editing to make it readable.
Hope that's of some help to you.