For question #1… you would need a Bash script (think ps -ef | grep -i java) or detailed substitution relevance to determine what Java processes are running on targeted systems and parse the results for parent process/service details.
For question #2… in the lack of an enterprise standard for *NIX partitions and installation path requirements for Java, this is difficult to answer. Does your organization have BigFix Inventory? If yes, then you could leverage the BFI REST API to collect those details.
For question #3… while rolling back Java is possible, it should be avoided by testing your applications extensively with the OpenJDK version you intend to deploy. That said, implementing enterprise standards for OpenJDK within your organization will help regarding this matter.