Ever since I changed the dbpath in /etc/mongodb.conf, MongoDB has not been starting automatically, nor using the new dbpath. Prior to
MongoDB 1.6 is very old and the latest production version is 2.2, which contains a large amount of bug fixes and enhancements since 1.6.
Am I correct that you haven't installed 1.6 via a package manager such as yum or aptitude? I don't believe there are packages for 1.6 at present afaik. Therefore, mongod is behaving correctly as you have not started MongoDB with a control script.
Please see this link on configuration file options.