In your case you know the size, don't you?
Yes
That's almost the same, isn't it? (there is just the max_prot parameter, which you can indeed add to the RPC above).
It's similar but I think the RPC should belong to the memory_object_proxy_* family intead of vm_* and be implemented in memory_object_proxy.c, so all logic related to proxies is in that file.