If you sometimes want to use some of the disabled features when using a broadband connection consider using two profiles which use common directories outside of the profile to store the messages. One profile would disable features as described below. The other could keep them enabled. That way depending upon which Thunderbird shortcut you use you can easily switch configurations with minimal side effects.

A bind mount is basically where you mount a given directory on top of an existing one. Suppose you have a RAID array where you have a partition mounted at /home2, containing some larger user accounts. If you wanted to remount /home2/user to /home/user (to sidestep issue #1), without the issues that come along with symlinks (it is not a directory, just a token that points to it), you'd do something like mount --bind /home2/bob /home/bob and the directory will then be traversable from both locations. The target folder must exist, same as any mount point. The end result is somewhat similar to a symlink, but instead of creating a special filesystem object, it utilizes the operating system's filesystem mounting machinery to do it, which makes it more transparent to running software. Tools like 'du' and 'find' will still be aware that they are cross filesystem boundaries, and will also behave as such if the bind mount is entirely within a given filesystem. Finally, as they're transient by nature (unlike symlinks), they need to be placed in fstab or some startup script to make them persistent.