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BUSTER install - CATCH-22

I wish to install Buster on a machine with essentially *NO* internet connectivity. I downloaded the DVD1 ISO on my primary machine [creates secondary problem of being up against data cap]. The installation, having put the DVD1 ISO on a flash drive, encountered no problems

*HOWEVER* I need several packages [gparted, tcl, tk] which are in the DVD image.

Synaptic is unable to install them:
  1. sources.list refers to a non-existent physical DVD.
  2. the default installation has no way of informing Synaptic
     that it should look for an alternate iso image.
  3. modifying sources.list was unsuccessful

I added the line
deb "file:/media/richard/Debian testing amd64 1/debian" buster contrib main

to the beginning of sources.list .

The Synaptic error message when attempting
   Edit --> Reload Package Information
> Download is performed unsandboxed as root as file '/media/richard/Debian testing amd64 1/debian/dists/buster/InRelease' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)The repository 'cdrom://[Debian GNU/Linux testing _Buster_ - Official Snapshot amd64 DVD Binary-1 20190327-09:46] buster Release' does not have a Release file.Updating from such a repository can't be done securely, and is therefore disabled by default.See apt-secure(8) manpage for repository creation and user configuration details.The repository 'file:/media/richard/Debian testing amd64 1/debian buster Release' is not signed.

This is unacceptable when installation via an iso image on a flash drive is becoming more and more common.

What package(s) to file bug(s) against?
Was the syntax of the line added to sources.list correct?
How to work around the permission issues in the error message?
   Ran into something similar when Squeeze was current.
   I haven't found my correspondence from that time to
   see if it covers current situation.