First question is about decentralization. What is off-chain relay in this case? Is that a centralized part of the system? Are there centralized parts of the system?
Off-chain relay is how parties find each other facilitate a trade. As a maker, I can create an order off-chain, but I still need to find a relevant taker who is interested in filling the order by sending it to the 0x smart contracts on-chain. Relayers pool together these off-chain orders and serve them up through a website, so that it's easy to find people to trade with. Note that this process is optional -- you can still trade peer-to-peer if I send you the order over email, SMS, etc. While relaying is centralized, relayers are non-custodial, so your funds are never at risk. We also have plans to make some parts of relaying less centralized in the future through 0x Mesh: https://blog.0xproject.com/0x-roadmap-2019-part-3-networked-...
Can this be used to implement more robust margin trading?
As for margin trading, yes, you can trade dY/dX tokens on 0x relayers, or combine 0x orders with any on-chain lending protocol such as Dharma or MakerDAO to get leverage.
What settlement speeds can be expected?
All trades currently settle on-chain, so the Ethereum block time is our bottleneck. Look for some updates on this near future :)
