I am sorry that you feel this way about the plugin. Please elaborate a bit more about the following statement.
the ONLY supported method is passing access token in a GET parameter
I am not aware of anything that this plugin has to do other than provide authorization to to WP REST API (authorize an access token). If there is something that you would like to see added to the OAuth2 side, please suggest it to IETF.
People are more than welcome to use OAuth 1 which is found by visiting https://github.com/WP-API/OAuth1. IT IS FREE. It is OAuth 1.0a which is not OAuth2 and thus anyone looking for OAuth2 SHOULD NOT use OAuth 1.0a.
I really wish you would of simply handled things a bit lighter and I would have been more than happy to bend the rules a bit.
Please keep this review based on the free version ??