Thanks. I opened a ticket with my provider and found:
That there is an admin screen I can use via VDeck to access the php.ini file and make changes. Very nice of them. Second, they took a look and suggested both setting for register globals and they also noticed there was no setting for php sessions. I will adjust those.
Note to all: check with your provider about the php.ini file. They can probably help you - most shared hosting providers do not allow SSH access or other major access like this and they are used to these kinds of questions. I should have asked them earlier. I was able to get the 1.01.5 invalid session error to go away, eventually. I'm not completely sure what did the trick. By the time the error went away, I had: created an entire second instance (which I will now use for testing) that captures it's own sessions locally (folder in the joomla directory based on another thread
http://forum.joomla.org/viewtopic.php?f=268&t=296527&start=0&st=0&sk=t&sd=a here in this forum) and reset the permissions on the original installation (production) to 777 temporarily so that I could get a valid login. I think it may have been something about loading the administration/index.php file during login that was causing some sort of problem. I had completely reinstalled joomla at a 1.0.15 level, including the installation directories, and attempted to login as clean installation. This of course bombed at the database part of the installation wizard, at which point I could simply reload the old configuration.php file and go on my merry way.
If you try this trick (permission settings), remember to set everything back to 755 and then go into administration (within Joomla), navigate to the global configurations control panel and set the configuration.php file to unwriteable. (I think that's 644 for you FTP wizards). Oh, and uninstall the Installation directories. You don't want those lying around.
At this point, I seriously need a backup and I'm working on that. But my access problem is solved, and I also have an upgraded Joomla installation and a second operation to use for testing. I never thought I would have to follow enterprise level techniques, but they sure come in handy.
Maybe I should write a backup section for the Joomla docs.