No. Only features that don't infringe additional patents can be added by the community. If Oracle owns patents covering a particular Java 7 feature, but never releases any GPL code implementing that feature, then the OpenJDK community cannot implement that feature.
Releasing code under the GPL does not give the open source community carte blanche access to all your future patents, perhaps unless those patents are necessarily infringed by the original GPL release.
Indeed. The GPL protection only covers OpenJDK as it is released now. However, Oracle would have to tailor the features in JDK 7 as to be under patents they own or licensed, but that are not used in OpenJDK.
And the OpenJDK folks can always implement around those patents.
Clean-room reimplementation are not though.
Harmony was created to have a non-GPL implementation (and was used by Google for that reason)