you dont have a diagnostic/test tool included
Such diagnostic tools are usually provided by device vendors. We might have something similar in support but it's used for troubleshooting purposes.
what happens when admins changes block size after/before writing?
New block size is set to the drive and used for all the further operations. Tapes written with previous block size value would require changing the value back to read the data properly.
verify/read test for every job if wanted by the user
Added your vote to tape verification request. Thanks!