No concrete plans for now. JS is complicated, and we don't have enough people to do that. Relying on a JS engine that's currently being used (which we know will get security updates) is much better than rolling our own, for now.
The js engine is separate from the rendering engine as far as I know. There might be some exciting new js engines written in Rust, but I don't think they would be part of Servo per se.
No, they plan on embedding SpiderMonkey like Firefox.
It uses SpiderMonkey.
If possible, will it be very complicated to use V8? Or is it just impossible?
It's not impossible, but I don't know why you'd want to use V8.
I would think a JavaScript engine would be done better on a different domain specific language than Rust. It seems that a virtual machine has a different set concerns to deal with than a rendering engine.
Rust isn't a domain-specific language. I don't see why it wouldn't be a good language to implement a JS engine in, from my experience.
Language VMs have to be very low-level and do a lot of "unsafe" things that Rust and such wouldn't like.