what is to prevent someone whos running a node to make off with the mixer coins? [...]

Thanks for the questions. It does seem like you're missing something. Although, it might not be your fault. The whitepaper is definitely out of date. We've done a lot of work at tweaking the trust model so that it can't be exploited. I'll try to explain how it works briefly, then hopefully if I get time I can revisit the whitepaper soon.

- Masternodes don't have any power over the transactions. They just coordinate the signing. All parties must sign in order for the transaction to be valid. So there's no way to cheat and take the money.
- Users submit collateral. At a later phase if a user doesn't provide the signature as agreed, the transaction will fail. Without colateral this could be done over and over bringing the system to a halt.
- Masternodes have the ability to take the collateral transaction if they wish, but it's paid to the bounty fund. So it doesn't benefit them, it just benefits the community. This removed the incentive to cheat and take the money.

There's no relying on pools at all anymore. Payments to masternodes are done with a voting system embedded into the blockchain. It would take 51% of the mining power to pay the wrong masternode, or another party (because the last few miners to solve blocks must agree on who should be paid)

Transaction currently require 3 parties to be created, so there's a short wait. There are no fake transactions to make that quicker, although this could be done. There's usually 5 or so transaction per 2.5 minutes, so the network should be able to function pretty efficiently under these requirements.

Hoping that helps . Thanks,

Evan