Anyone who got cancelled by world choice looked at EU261? Just writing a letter but guidance says it’s flight operator rather than who you booked with. Given Ethiopian effectively offered their plane but WCS ended up not taking it not sure who to write to!!
I'd initially write a letter / email to WCS requesting compensation in accordance with EU261 and see what comes back. They are bound to reply that they are not responsible , but see what the response is initially . They may say that Etheopian Air are responsible , so it may be worth sending a similar compensation calim Etheopian Air at the same time . Again , expect Ethiopian to reject your claim .
See what comes back and if it's the expected rejections , refer your complaint to the CAA . I've dealt with the CAA a few times and they're pretty good.
This is not your usual claim under EU261 , so will be interesting to see how it develops .
An alternative would be to put your case with a fight delay compensation specialist . They'll know excatly where you stand , so if they think it's iffy , they'll let you know . However , bear in mind that if you do pursue a claim in this way , they will take a hefty commision for it if you're successful. .