Super admin access to a forgotten WP instance?

I have inherited a WordPress instance backed by a MySQL database, hosted on a bare bones server. I have SSH access to the server, and I can even log in with an old set of credentials to the admin panel. But the login I have does not allow me to install plugins, which I now must do.

WP site after login keeps redirecting to looped url

My site Example.com keeps on redirecting to following url https://example.com/?redirect_to=https%3A%2F%2Fexample.com%2F%3Fredirect_to%3Dhttps%253A%252F%252Fexample.com%252F%253Fredirect_to%253Dhttps%25253A%25252F%25252Fexample.com%25252F%25253Fredirect_to%25253Dhttps%2525253A%2525252F%2525252Fexample.com%2525252F%2525253Fredirect_to%2525253Dhttps%252525253A%252525252F%252525252Fexample.com%252525252F%252525253Fredirect_to%252525253Dhttps%25252525253A%25252525252F%25252525252Fexample.com%25252525252F%25252525253Fredirect_to%25252525253Dhttps%2525252525253A%2525252525252F%2525252525252Fexample.com%2525252525252F%2525252525253Fredirect_to%2525252525253Dhttps%252525252525253A%252525252525252F%252525252525252Fexample.com%252525252525252F%252525252525253Fredirect_to%252525252525253Dhttps%25252525252525253A%25252525252525252F%25252525252525252Fexample.com%25252525252525252F%25252525252525253Fredirect_to%25252525252525253Dhttps%2525252525252525253A%2525252525252525252F%2525252525252525252Fexample.com%2525252525252525252F%2525252525252525253Fredirect_to%2525252525252525253Dhttps%252525252525252525253A%252525252525252525252F%252525252525252525252Fexample.com%252525252525252525252F%252525252525252525253Fredirect_to%252525252525252525253Dhttps%25252525252525252525253A%25252525252525252525252F%25252525252525252525252Fexample.com%25252525252525252525252F%25252525252525252525253Fredirect_to%25252525252525252525253Dhttps%2525252525252525252525253A%2525252525252525252525252F%2525252525252525252525252Fexample.com%2525252525252525252525252F%2525252525252525252525253Fredirect_to%2525252525252525252525253Dhttps%252525252525252525252525253A%252525252525252525252525252F%252525252525252525252525252Fexample.com%252525252525252525252525252F%252525252525252525252525253Fredirect_to%252525252525252525252525253Dhttps%25252525252525252525252525253A%25252525252525252525252525252F%25252525252525252525252525252Fexample.com%25252525252525252525252525252F%25252525252525252525252525253Fredirect_to%25252525252525252525252525253Dhttps%2525252525252525252525252525253A%2525252525252525252525252525252F%2525252525252525252525252525252Fexample.com%2525252525252525252525252525252F%2525252525252525252525252525253Fredirect_to%2525252525252525252525252525253Dhttps%252525252525252525252525252525253A%252525252525252525252525252525252F%252525252525252525252525252525252Fexample.com%252525252525252525252525252525252F%252525252525252525252525252525253Fredirect_to%252525252525252525252525252525253Dhttps%25252525252525252525252525252525253A%25252525252525252525252525252525252F%25252525252525252525252525252525252Fexample.com%25252525252525252525252525252525252F%25252525252525252525252525252525253Fredirect_to%25252525252525252525252525252525253Dhttps%2525252525252525252525252525252525253A%2525252525252525252525252525252525252F%2525252525252525252525252525252525252Fexample.com%2525252525252525252525252525252525252F%2525252525252525252525252525252525253Fredirect_to%2525252525252525252525252525252525253Dhttps%252525252525252525252525252525252525253A%252525252525252525252525252525252525252F%252525252525252525252525252525252525252Fexample.com%252525252525252525252525252525252525252F%252525252525252525252525252525252525253Fredirect_to%252525252525252525252525252525252525253Dhttps%25252525252525252525252525252525252525253A%25252525252525252525252525252525252525252F%25252525252525252525252525252525252525252Fexample.com%25252525252525252525252525252525252525252F%25252525252525252525252525252525252525253Fredirect_to%25252525252525252525252525252525252525253Dhttps%2525252525252525252525252525252525252525253A%2525252525252525252525252525252525252525252F%2525252525252525252525252525252525252525252Fexample.com%2525252525252525252525252525252525252525252F Answers: Thank you for visiting the Q&A section on Magenaut. Please note that all the…

Can’t extend some core classes

I am relatively new to wordpress development, I am trying to extend ‘Walker_Category_Checklist’ class found in /wp-admin/includes/class-walker-category-checklist.php to customize it for wp_terms_checklist() or wp_category_checklist() but when I try to require_once a link to my extended class in the child-theme function.php I get this error and site not loading!