[rob]

Yes, WordPress gives administrators the ability to change their theme, edit theme and plugin code (which can be disabled in wp-config.php in two lines and should probably be standard if you're setting up a WordPress website for somebody), and make changes to their website — that's why their administrators.

You should probably think about giving certain users like 'content people' the 'Editor' role if you don't want them doing administrator stuff like editing the theme file directly.

I'd imagine most CMSs where you give your client full administrator access can cause issues with them doing things they shouldn't, like blindly upgrading plugins.

Which SEO plugin gave you that 500 error? Most WordPress plugins have a 'Requires PHP' header that specifies the minimum PHP version required and refuses to install otherwise.