Ubuntu – Disable auto-mounting in Ubuntu 16.04

Using any mount system, you want to avoid situations where Nautilus lists the directory containing a mount that may or not be mounted. So, with autofs, don't create mounts in, for instance, /nfs. If you do, when you use Nautilus to list the 'File System' it will try to create whatever mounts should exist in /nfs, and if those mount attempts fail it takes minutes to give up.

So what I did was change auto.master to create the mounts in /nfs/mnt.

This fixed the problem for me. I only get a long delay if I try to list the contents of /nfs/mnt, which I can easily avoid.

External media/drives mounting is handled by udisks2 on most modern distros. I don't think there's any trivial way to change the default mount options as they are hard-coded (see FSMountOptions in udiskslinuxfilesystem.c) that is, they're not configurable (at least not yet¹). Your options are quite limited: unmount the partition and remount it with different mount options (unless you're willing to patch the source code or write your own automount tool).

As to your other question:

I think one may be able to disallow mounting by type, though, by the looks of the rules?! When I insert a USB (3.0) thumbdrive or HDD all ext[34] partitions get mounted (I wish they weren't) and the user gets a graphical prompt for any LUKS partition to unlock. I wish to disable both. A user may have FAT drives but others may only be mounted by root.

You could use a udev rule to ignore all USB thumbdrive partitions except vfat ones. Create a new rule file e.g. /usr/lib/udev/rules.d/90-ignore-nonvfat.rules with the following content:

SUBSYSTEM=="block", ENV{DEVTYPE}=="partition", ENV{ID_BUS}=="usb", ENV{ID_FS_TYPE}!="vfat", ENV{UDISKS_IGNORE}="1"

(replace UDISKS_IGNORE with UDISKS_PRESENTATION_HIDE if your distro uses udisks1).

^{1: see FreeDesktop ML for a proposed patch (and a long discussion).}