Should test / verify commands & --backup-test-samples support percentage parameter?

Yes, with a caveat.

This How-To guide should help with listing (and purging) the files that are “broken” by the apparently corrupt dblock files:

The caveat is that this process detects what files are broken and lists the contents of those files. I don’t know that there is a way to list affected files for specific dblocks (@kenkendk or @Pectojin might be able to answer that).

If you try list-broken-files please let me know if it works with the “failed to decrypt” related dblocks.