Many a times it is seen that gamblers post a lot of complaints but can't provide proper proof, thereby ruining the casino's reputation for no reason. Everything I read in this thread. OP has not been able to prove ownership of his account. And he managed an account with such a large fund, without properly storing the account information! If someone owns an account he will at least have email access if he fails to do so in other ways. So I think the original owner can be verified with the main email address of that account. I will wait to see if this complaint is resolved.
Duckdice doesn't have solid clear reputation in the past though but there consistency to operate there casino until now is what makes them solid.
Reading the OP case many times including his rellies. He is claiming that his account is already KYCed even though he didn't complete this before which is the reason why his new KYC to prove the ownership is being rejected by Duckdice. Its very hard to judge on this case because there is no evidence for both side for there claims. You can check his statement on this link
https://bitcointalk.org/index.php?topic=5430960.msg61499918#msg61499918 well before there was like 3 red flags active on duckdice.io profile but it seems they have managed somehow to remove them. past of duckdice was worst yeah