Thanks for using Cloudera Community. Based on the Post, HBase Master isn't progressing on StartUp owing to "hbase:namespace" being not Online. Your Team tried to assign the Region via HBCK2 yet the issue persists. Additionally, the RegionServer on which the "hbase:namespace" Region is marked for assignment is under Live & Dead as well.
The approach used by your Team is correct with respect to using HBCK2. When your Team run the HBCK2 assign command, It should return a PID before terminating. The Output would be like , assuming "10" is the PID. Review the HMaster UI for the concerned PID & review the State.
When an RS has Live & Dead Status, Review the StartUp Code for the Live & Dead RS. For Example, the RS "hbase-test-rc-5.openstacklocal,16020,1469419333913" StartUp Code is "1469419333913". We expect the Live RS StartUp Code would be recent & Dead RS StartUp Code would be Older with the "hbase:namespace" Region being tried to be assigned to the Live RS. Use the Epoch Converter via Link  to convert the StartUp Code to Human Readable Timestamp.
If the PID State is fine with Region being assigned to the RS with Live StartUp Code, Review the concerned RS Logs to confirm if the Region Assignment has been received & any issues with opening the Region. Accordingly, Proceed with the troubleshooting.
Do review & let us know your observations.