On further investigation, looks like an issue with permissions on install dir. There may have been changes to UAC configuration on the Windows 7 OS where Splunk was installed (group policy applied), found a mongod.lock file while no mongod processes were running, deleted and rebooted, subsequently found more access errors for journal files, manually granted administrators full control over "C:\Program Files\Splunk\var\lib\splunk\kvstore" and files/subfolders, looks to be working now.
