IMO you got three agents who clearly don't know the policy.
The first two agents in the club were wrong. But do you really expect them to know the ins & outs of SDC? They don't likely know them.
The DM line should have been able to get you into F as well. You had a horrible agent there.
This is all assuming you know there was fare class availability in the cabin and not just looking at three open seats on a seat map because the seat map is not an indicator of what DL is willing to sell on the flight. If they were willing to sell seats on F then you should have been able to get one.
The best solution: HUCA.
-RM