No its not entirely imposible to have a slow leak on an oil filter housing. But given the situation you or your inlaw would have noticed the leak sooner, and called them to let them know. Given that you popped out of nowhere 5000 miles later, the car looked at elsewhere ( i understand he is miles apart) kinda make it hard for the SM to see where your coming from.
Being I speak from experience behind a service desk, Im not saying your wrong, but just saying I understand why he didnt think it was their fault. I too woukd question why wait so long to adress the issue. It very well possible it had a simple leak elsewhere, and the other SM mis understood the tech, and mis informed you. Not stating this is the case.
In any regards I would have given you the full refund given your service records, but me in your shoes, I probably would have just left it alone, and not raise hell about it.