Here's good news around Sierra the mounting challenges:
The original Unix automountd works again as it is supposed to work.
Do as root this:
Add into your /etc/auto_master for example this line to a static mount map. I called it "Media", but you can do anything you like.
/etc/auto_master:
/Volumes/Media auto_media
then make a file /etc/auto_media with the entries of your mount-points, these are two in example for picture and music. My Synology NAS fully supports apple files system, but you can do this on any file server and pick other file system. See man auto_master.
/etc/auto_master:
Bilder afp://yournasuser:naspw@Media/Bilder
Musik afp://yournasuser:naspw@Media/Musik
Also make the mountpoint folder manually mkdir /Volume/Media and reload all with automount -vc
If you now click as a USER in Finder to the /Volumes/Media folder, Finder will automatically show the subfolders Bilder and Musik.
If you click into one of those, automountd will automatically mount the volumes AS the USER that was requesting it and NOT as root.
This is the key to the solution!!!
Check out the mount status, it will show the following:
Type mount
...
map auto_media on /Volumes/Media (autofs, automounted, nobrowse)
//yournasuser@Media/Bilder on /Volumes/Media/Bilder (..automounted,.. mounted by yourMacUser)
Please note the "mounted by Username" at the end of the last line above.
I realized that the mkdir /Volumes/Media is persistent on one Mac, but all folders in /Volumes are wiped out on another Mac. In this case you have to execute after a the reboot a script that creates the folder and reloads the automounter.
Something like this:
#!/bin/sh
mkdir /Volumes/Media
automount -vc
I hope this works for everybody.