I restarted OH2 and am now back to the refreshThing() exception='null' error though my changes are now reflected in ZM, so that is good.
It looks like the other issue, where it is just setting the values to UNDEF may be because the Autodiscovered things weren't configured correctly, even though the thing definition in karaf showed the bridge was set correctly, the log states:
Thing 'zoneminder:monitor:756ae518:monitor-1' has Bridge 'null' defined
so the connection must not be stable. After a restart it doesn't cycle through the setting the values to UNDEF anymore, just:
[BRIDGE (756ae518)]: Method 'refreshThing()' for Bridge zoneminder:server:756ae518 failed for thing='zoneminder:monitor:756ae518:monitor-1' - Exception='null'