Wow,
Too many issues to be a one answer.
Could be a bad upgrade!(meaning a corrupted file or uploaded over a file you should not have)
Could be a file/folder permission issue.
Could be a cache issue related to all the above.
Installing in a new directory is not a issue.
Copy over your old forum.
Edit the includes/config.inc.php for new paths.
Get it running then upgrade to 7.5.7
Probably what you needed to do as a test anyway.
You would treat it the same as moving to a new host.
I don't suggest using the old dbase with a newer version not installed without the upgrade option.
Bad habit to get into.
Because some upgrades will modify the database depending on what version you are upgrading from.
