On the legal question:
As of writing, the Contributing page states:
"License
Submitting changes implies granting project a permission to use it under an appropriate license."[0] "license" links to [1] which states (in part):
" * Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
" followed by standard liability disclaimer.
There may well be a problem agreeing what "appropriate" means. There's an argument that given the link, appropriate could mean a license that allows redistribution in the same way as the current license - which would raise interesting questions for redistributing the nginx plus version.
Certainly it's not a simple assignation of copyright to NGINX. I can easily imagine a lawyer having a heart-attack when reading the original conditions of submitting.
[0] http://nginx.org/en/docs/contributing_changes.html
[1] http://nginx.org/LICENSE