1. DL should have had control of the ticket by the time you arrived into SEA. I'm surprised that they couldn't fix it, but perhaps the system thought that you no showed and cancelled the rest of the ticket.

2. Given 1., do you have more segments on the ticket/PNR? If so check with KLM that the rst of your travel hasn't been cancelled.

3. There's an amazingly extensive and informative thread in the BA forum on EC261 rules. AFAIK since you arrived into SEA late (as part of an exEU ticket/EU carrier), EC261 rules should apply and give compensation.

4. The USA DOT rule would only give a percentage of the fare allocated to the segment where boarding was denied, which probably won't be much even if 400% applies.

5. Also, note that the USA DOT rules only apply if your case satisfies their criteria for IDB, which included the requirement that the flight must be oversold. I suspect that you may have lost the seat because standby passengers were cleared onto the flight, although it would help if you could tell us how much time there was between scheduled departure time and when you reached KLM.

6. DL can argue that you didn't have a ticket, even if this was due to a system error.