No. Partial updates are not supported. It might work for a while for a couple of packages, but then you will lose control of dependencies or break the install.
In theory, all that is needed from unstable
for this purpose here
instead of a full update
is
linux515 from the core repo of unstable branch
and the corresponding host modules from the extra repo of unstable branch
for the virtualbox version also from unstable
(or any different kernel version and corresponding host modules)
It should be enough
since the virtualbox from unstable seems to work for you.
In essence - just a kernel update to the one from unstable - in order for the modules to work.
I’d make sure to have at least one other kernel (different version) to boot from and verify that it works before doing this.
Thank you so much for driving me through the maze, it worked!
I just upgraded the kernel and its modules, tested after a reboot and went back to stable branch. Now i can see and access USB devices within VirtualBox.
Thanks also to Ngr who pointed out this more than new version of Virtualbox 
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