1. The plan is precisely to inherit the Linux hardware support through the DDE layer...
2. Running something as root is definitely an issue. There are all kinds of breaches here and there, and CVEs flowing. Xorg tries to *not* run as root for instance.
3. Linux is mature and rock-solid, yes. At least what is mature in it. New FS implementations some ext4 still have an experimental phase, in which encapsulating it can be very useful for debugging.
4. LD_PRELOAD is full of potential for horrid issues. You need to distinguish between the application FDs and the FDs you are running in your library, which is very bugprone. FUSE does not implement quite a few things. I've just realized that you can give a tun/tap to a user, so that the application running it can be non-root. This is indeed a good thing. That however does not permit to implement e.g. dynamic table routing, which vpn clients often need (and I do not like running as root).
5. The virtualization part (in particularly full virtualization) is a hammer for a fly. Containers & namespaces are indeed more like the Hurd approach, we'd say "at last there are such facilities in Linux". They however have a hard time getting everything integrated mainstream, I'm not sure they will complete it. The Hurd, from the start, uses indirection, and is thus already flexible.
"change it to use (unmodified) Linux instead of Mach/L4/whatever as a "supporting microkernel""
I guess here too you missed the DDE layer plan. That plan being considered, Linux has not much to bring compared to Mach: the IPC interface would have to be completely rewritten. The FUSE part is already in the plan, there is some embryon of code.
"assuming that there actually are any useful parts that can be salvaged that way."
Of course, if you just take Linux, then you have a Linux system and there's no use in doing that :)
Our plan is rather to take bits from Linux, such as drivers & FS implementations, and integrate them in the Hurd landscape.
I'd like to repeat again: we don't plan to replace Linux. I guess there might be some difference in what one can mean by "succeed".