Powering only a subfolder with WordPress on a LAMP stack with an nginx reverse proxy

If you are still looking for a solution, your nginx configuration should look like this in order to achieve what you described…

server {
  listen 80;
  server_name yourdomain.com

  root /path/to/yourdomain.com;
  index index.php index.html;

  location / {
    # directives to handle static site
  }

  location /sub {
    # directives to handle WordPress
    try_files $uri $uri/ /sub/index.php?$args;
  }

  location ~* \.php$ {
    # pass to Apache backend
  }
}