How will macOS behave when creating folders in /private/var/folders to mount a NFS share with?
No.

This is a system directory that contains things like logs, databases, your sleep image, etc.  It's not a directory that you should be mounting file shares to.  The purpose of the /private directory has already be discussed in the following posts:


What&#39;s the &quot;Private&quot; directory in OS X for?
Snow Leopard *private* folder?


Even the automount service doesn't mount shares to that directory, it's mounted in /Volumes.  

Your best bet would be to either mount the share in a shared user space (i.e. /Users/shared) or within the users own home directory.