First, it is for KL to prove an "extraordinary circumstance" such as weather. While the local weather in ATL or AMS is irrelevant as weather enroute matters just as much,if not more, routine cancellations of the same flight on multiple consecutive days make it hard for KL to prove. The sole determinant here is whether the departure time from ATL was, earlier than 20:25, e.g. 2 hours earlier than the originally-scheduled departure time of 22:25. If so and your husband arrived at his final ticketed destination which appears to be somewhere in Norway earlier than originally ticketed, he is due 50% of the standard EC 261/.2004 cancellation compensation. That would be EUR 600 for a Type 3 flight, this EUR 300 in cash (equivalent) for this change.
Your husband should file his claim with KL and stick to the relevant facts. Leave out all of the back-and-forth over the course of the day and focus on the originally scheduled departure and arrival times and the actual times,
Second, your husband accepted a voluntary settlement for denied boarding by DL. While a $400 voucher is extremely low and I would have insisted on at least $1,000, if that is what he chose to accept, that is what he has accepted. Bear in mind that had he been involuntarily denied boarding, he would have been due 400% of the value of the segment in question up to $1,350 and that amount would have been payable in cash. If your husband can make use of the DL voucher, it is at least something.