Interesting about the Iris scan, that would indeed worry me. But I fail to see what it has to do with the check-in machines. Presumably somebody can check in with a check-in agent and still send somebody else to the gate, yesno? Only ID check at security can prevent that.
As for (b), I presume you refer to suitcases travelling alone before the passenger has travelled, not stray luggage that has to be sent after the traveller because somebody messed up or connection times were to short. Also interesting but not suprising. And nothing a check-in agent can do against somebody who checks-in luggage and then does not show up.
Bottom-line for me: There is no security difference between a check-in agent and a machine. The agent would take the luggage and check it in. Same as the machine. Whether or not that person, a different person, or no person shows up at the gate is not in the hands of the check-in agent. Pretending a check-in agent is more secure than a machine is just using the fear of travelers to secure jobs. Rather have more, better paid and trained people at security screening so it is better and faster instead of adding folks at the check-in