First and foremost, forget about EC 261/2004 compensation until your in-laws have handled their rebooking. It is presumably not time sensitive and that can be handled once they are safely back home (or wherever they wish to be).
EC 261/2004 provides the following rebooking options:
A. Cancel for a full refund.
B. Rebook under similar circumstances.
C. Rebook at a later date.
While SK is free to rebook on other carriers, it is not obligated to do so and nothing in the Regulation requires it to do so. I would also strongly counsel against a reroute at this point. Even if they can be rerouted as there are numerous other options, the prospect of being stuck at an intermediate city on one side or the other of the Atlantic is not appealing unless one has important meetings on the other end (and even then).
If you are intent on getting them to IAD, you should do the legwork and propose options to SK. The easier you make it by feeding the agent flight numbers, the better.
My advice is to fly on the 20th if at all possible.