Congrats, Emma!
. So due to environment check and plugins issue, site was never installed/upgraded and that's why we kept getting sent to /admin/.
Now some suggestions ...
Do you still have the issue of a plugin you cannot un-install?
There is CLI script in admin/cli/ called uninstall_plugins.php
Run without parameters will bring up help screen from which you an choose/try the options.
Might have to manually remove the offending code folder/directory and moodle will complain, but we know why it's 'missing from disk'! Press on ... go forward anyway.
Check environment using cli checks.php
Try the upgrade via command line - in cli there is a upgrade.php ... it does do an environment check prior to attempting. If there are errors ... fix 'em! Then try again.
All the above can be done when site is in the maintenance mode - which can be turned on and off with another cli script:
php maintenance.php --enable
php maintenance.php --disable
'SoS', Ken