It seems I found a (temporary?) solution to the 3rd. problem which is the one most concerns me because it impacts the normal use of my USB system.
Once the filesystem is corrected using another distro with the e2fsck command, when I boot again my USB persistent system, and before to halt the system, I execute the sync command before to execute the shutdown command.
There are still some I/O errors in several sectors of the USB device, but, in the following on boots of the USB system, it seems to work better and the non-ending sequence of errors on the EXT2 filesystem is reduced to only 6 to 8 messages and the system seems to work properly.
Anyway, if anyone is able to give me a better and more definitive solution, not only for the 3rd. problem, but also for the other 2 ones, I would very much appreciate it.