Stunna of course can see everyone's seeds. He can't manipulate your bets due to the way that the site's provably fair algorithm works. If he tried, he could be caught, and publicly outed. It's not worth the risk. The only real risk is that he could make an account and play on it knowing the seeds, and use the seeds to win. But he would only be winning from himself (and his private investors, assuming he has any). It's not a concern for players.
I am not a negativity person but suppose you do it right now . It means you can empty all of invested Clams in Just-Dice ?
Yes, he can, and so can anyone who runs an investment site. Investors just have to trust them with it. I won't actually be surprised in any of the site owners actually do this. Its almost impossible to say with confidence, that someone is not doing it.
But I suppose investors understand this risk before investing.