"OK, so any 7 of 10 investors can generate the roll"?
Well, in that case I'll just try various sets of 7 from 10 until I find one that makes me win...
Any way you roll it, I can't see a reliable way of stopping myself from being able to cheat while also having the site be able to process player rolls quickly.
I think this warrants more consideration. It would go a long way if you could prove that the site is fair for investors too.
How about having 5 or so servers, but you don't get to decide whether to use their seed or not. By default, all of them would be used, which would be most of the time really. But in case one goes down, halt the game for a bit and have a status indicator on the site show that a server is down. Game could then continue, or someone could wait for all servers to be online again if they so wish. Stop the game completely if less than 3 servers are online or so. The servers should also have some independent way to verify whether they're running or not, such as a webpage. These additional servers would be run by big investors or some other relatively trustworthy party, and perhaps you could fund them from your profits, would probably not be a lot. Some of them could also be hosted on different providers, to prevent a VPS service employee or a hacker to gain access to all of them.
Would such a solution work? It would add some complexity and maintenance costs, but the added investor confidence would be well worth it in my opinion.