no, actually the error you're getting:
_XSERVTransSocketUNIXCreateListener: ...SocketCreateListener() failed
_XSERVTransMakeAllCOTSServerListeners: server already running
is due to another X server already using the same display number, as per source and FAQ mentioned in comment-206473.
An existing lock file is a pretty good signal that another X server is using the same display number, the only way I can see a lock file remaining if the X server process crashed or was killed with SIGKILL. And as can be seen in the X server source code in that case X server tries really hard to remove the stale lock file and to launch at the specified display number.
I think that the handling of X server lock file is best left to the X server. I could add the functionality to the Xvfb plugin to kill already running X server detected by the lock file. But as you can imagine this would result in erratic build behavior as concurrent builds would terminate each other, or user's X sessions would unexpectedly terminate.
The zombie process handling is automatic termination of Xvfb processes left by slave disconnect or Jenkins master crash. So if your slave disconnects, for example due to a network issues, the build fails and no Xvfb process termination could be performed as the slave cannot be contacted to execute the process termination. For these cases Xvfb plugin keeps a list of Xvfb processes that it started, and on slave reconnect it goes trough the list and terminates the leftover Xvfb processes and removes the temporary frame buffer directory used by the Xvfb. This is done automatically on Jenkins master startup, and on slave reconnect.
I think that in your case you need to make sure that there is no overlap between display numbers, which could be caused by using the same offset for more than one job running on the same slave, using multiple slaves per physical machine or running X servers on the display numbers used by Jenkins. The easiest way to have non overlapping display numbers is to use the 'Let Xvfb choose display name' option and have that done automatically by the X server.