I don't understand why sshd quits without any error messages. Perhaps you can try one last time using the "-d -d -d" option to increase the debugging output. Maybe you can also see something in the kernel log using the dmesg command. Otherwise, I'm out of ideas. Sorry that it has been so much trouble for you.
Someone had got vsftpd to work on pocketbook devices in the past. You can get the package at:
https://www.mobileread.com/forums/sh...69&postcount=8. Maybe you'll have better luck with that.
P.S. That version of vsftpd-launcher.app requires a BACK button to quit. So, I hacked a new version that also quits if you long-press the NEXT or PREV buttons, since touchscreen devices don't have BACK buttons anymore.