Just as an update to anyone running across this...
I did a new install of 2.31.15 at at a nearly identical site (again, without a database), and the manager page is working for me again!
The one new step is, as you mentioned above, in config.php I need to change ADMINUSER and ADMINPWD to something other than the default "fop2admin". Once that's done, I am able to login to the manager page and all the buttons show up and I can save setting normally, etc.