@hokagetonton: That very much sounds like the date/time is messed up on your device(s). If you see the placeholder screensaver, cover mode is enabled, so you're good on that front (and autoreboot is irrelevant here, even more so on a K5, where it's fairly useless).
Note that Calibre isn't magic, you actually have to re 'Send To Device' updated books for the metadata to take
. [That said, except on very broken books, you should be okay with straight-from-AMZ books].
On the K5, you'll have to keep a book open for ~10s before the cover is locked on (should be faster on a K4, on the other hand). If your device is extremely busy (indexing), then the timing can go haywire. Check that it's not busy indexing, then try again.