403 and/or 404 errors when submitting a form are usually generated by an apache module called Mod Security. Almost all webhost providers have this enabled on their servers to protect their users. However some webhosts are using strict rules to prevent attacks so this is why it throws all these errors. You can contact your webhost to white-list your domain from using these rules.

This module essentially prevents server to accept sensitive html tags like <iframe> or <script>. This modules may also prevent URLs as a parameter in the site URL. E.g. index.php?url=http://google.com will cause the same error.

Yes. However the only issue might arise from URL rewriting. The following code has been tested by me and a couple of buyers and seem to work fine. Please note that this code must be added to nginx.conf and you will need restart your server for the change to apply. The code below should be added within the server { } block.

 if (!-e $request_filename)
    { 
        rewrite ^/admin/(.*)?$ /admin/index.php?a=$1 break;
        rewrite ^/(.*)$ /index.php?a=$1 last;
        break; 
    }

Please check out this video for more info https://www.youtube.com/watch?v=S2XqNLvGK80