Those people who claimed that they did get their payout must be some part of their plan or the site owner's shady tactics to made people think that their legit. Actually, there are sites like 999dice as you have accused of scamming gamblers that let the gamblers deposit and withdraw for small amount but when it comes to huge amount the withdrawal would be always pending and no payment received.
Yes, that's just like you said. They will keep your withdrawal "pending" for long time, and then when you are already unpatience of waiting your money/ feel bored, you will "play/gamble" those for a while and lose your money in the end.
Before making an accusation be better to send a message to the Admin to processing your withdrawal because i see your your withdrawal status still pending and it's still listed in your withdrawal history.
Before making any suggestion, you should try to search information regarding this site.
https://bitcointalk.org/index.php?topic=5153720.msg51448209#msg51448209And here is theymos's opinion about 999dice
Seems pretty plausible, though you're right that it can't be proven. Still, people should stay away from 999dice.com until they change the way they deal with seeds to rule out this sort of tampering.