Debian “Jessie” 8 with mongo-org installed using apt-get.
Did you use the installation instructions available on Install MongoDB Community Edition on Debian? If not, you may want to follow the instruction on that page since it will install the latest MongoDB version for you.
Verified with mongo shell: all config servers are reachable and rs.status() shows that the configDB cluster is functional
Could you share the output of rs.status()? How about the shard servers?
Mongos starts and stays running using mongodb user account, but
“netstat -plnt” shows that it’s not listening on any port and nothing is
listening on port 27017
This sounds like mongos did not start properly. Could you verify using ps aux that the mongos process is running? If not, there should be an error message in /var/log/mongodb/mongos.log.
Verified with netstat: Standalone mongodb runs under mongodb user
account and “netstat -plnt” shows it’s listening on 27017
Do you mean that you tried to start a standalone mongod to verify that a process can listen to port 27017?
You may want to peruse the procedure outlined in Deploy a Sharded Cluster page for hints of what went wrong.