Question 1: Once my cloud provider has upgraded, will this feature be automatically available to my existing environments?
This feature will be automatically available for existing environments after upgrade.
Question 2: Would it be possible to patch my 1.9.2.1 environment now? Is only the jar below required, and would it be compatible with my 1.9.2.1 env?
I can not say for sure, but you can try to patch your environment yourself.
http://www.sendspace.com/file/eo1ia7 (jelastic-gc-agent.jar)
Put this file to /lib folder by the dashboard and add such line in variables.conf:
-javaagent:/opt/tomcat/lib/jelastic-gc-agent.jar
I will be glad if this help you.