1 - https://github.com/servo/servo/blob/master/ports/cef/core.rs...
It's hard to say about single-process mode; it'll stay maintained as long as someone's willing to do the maintenance. There isn't anything that's inherently multiprocess about the architecture (for example, there are basically no global variables anywhere in Servo), in any case, so I don't imagine it'll be hard to keep it running. I don't want to make any promises, though.
In the Chromium Multi-process Architecture design document[0] the benefits boil down to security and stability. While memory safety doesn't guarantee secure logic, it obviates a large class of bugs that a multi-process architecture is designed to counteract. While memory safety doesn't prevent deadlocks, it does completely prevent data race conditions.
[0]: https://www.chromium.org/developers/design-documents/multi-p...
Remember that the goal of Servo is not just to be as safe as modern engines, but to be far safer (while also being twice as fast!).
A worthy goal! However, I'm still concerned. If the project is sufficiently secure without multi-process, adding layers only increases the surface area for attack vectors. Sandboxing not only increases the surface area of your (now) multiple processes, but also introduces the OS layer as an arbiter of communication between them, which we can safely assume does not have memory safety guarantees (though is probably quite hardened).
Of course, the developers would not claim that Servo is "sufficiently secure" right now, especially since (as brson mentioned) it still includes C & C++ source. My point is, sandboxing can be a huge win in terms of security, especially for now; but any complex feature has tradeoffs and the efficacy of those tradeoffs may not continue to hold through the future and should remain open to consideration.
I also don't understand "If the project is sufficiently secure without multi-process, adding layers only increases the surface area". The point of layers is that each one needs to be breached in order to craft a full exploit. If one layer is sufficiently secure without multi-process then in the worst case your defense-in-depth is merely redundant, regardless of the attack surface of the remaining layers (though obviously "sufficiently secure" is fairly impossible to prove).