Firstly, thank you for taking the time. Verbose assistance is nothing to apologize for.
I have to say that I am not at all eager to wipe out my whole Drupal installation with all of its modules, configurations and content just to make Ubercart work properly. This installation was made from the latest Drupal core, and ran perfectly until I started wrangling with Ubercart. It shouldn't be necessary to reinstall Drupal, unless Ubercart has damaged the database. If it comes to that, then I'm willing to give this one last shot.
For some reason the "variables" table doesn't exist. Did you delete it on accident? The first error looks like it's because the variables table isn't there, so it can't delete the rows that start with "uc_notify".
I should have made it more clear that the various warnings I listed do not continue to come up---they simply came up as I was going through the process of completely removing Ubercart to reinstall it. I did try to delete anything that looked like an Ubercart variable before reinstalling.
The variables table is fine. There are no entries beginning "uc_notify_" in it now, but then I haven't re-installed the Notify module. That message came up while I was uninstalling Ubercart, but before I manually deleted anything from the variables table. I would say there is a problem with any module if it is unable to uninstall itself without producing warning messages. That goes for both the Notify and Store modules.
The last warning messages came up on a product page that I never deleted when I removed Ubercart. I have since deleted that product page and re-created without those messages coming up.
So all in all, what I'm seeing is a lot of mess left behind by the previous Ubercart installation that screwed up the new installation. I would much rather be able to uninstall Ubercart completely... and cleanly! Shouldn't I be able to do that?