Also fwiw in DX12 even if parallel submission is supported, what's going to happen is N user threads are going to submit to 1 driver thread that then submits to the actual GPU.
And yes, ordering is needed here.
I can see that it might be not a major issue if time that takes it to fetch from the queue is way smaller than time it takes to execute some command [buffer]. I.e if queue manager will fetch and push commands to the queue, returning to fetch another without waiting for the first to finish, then it will work fine (it will have to handle GPU saturation though). But if it will be blocking - it will be some bottleneck which won't let using hardware fully.
Vulkan is basically a rebranded Mantle. I suspect you're just mistaken about Mantle, but as that API is not (yet?) open we don't really know if this was a change Khronos made when adopting Mantle or if this is just how Mantle works as well.
DX12 works the same as Vulkan, though, with a single submission thread ("The only serial process necessary is the final submission of command lists to the GPU via the command queue, which is a highly efficient process." http://blogs.msdn.com/b/directx/archive/2014/03/20/directx-1...)
It shows that multiple queues can be used for one GPU. I guess Vulkan can allow the same but it's not clear from that diagram.
Vulkan does support multiple command queues. And there can be several queues of the same type for one GPU.